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BIOLOGY

UBRARY

The Natural History
of Animals

o
u

The

Natural History
of Animals

The Animal Life of the World in its various
Aspects and Relations

BY
J. R. AINSWORTH DAVIS, M.A.

TRINITY COLLEGE, CAMBRIDGE

PROFESSOR IN THE UNIVERSITY OF WALES, AND PROFESSOR OF ZOOLOGY AND
GEOLOGY IN UNIVERSITY COLLEGE, ABERYSTWYTH

HALF-VOL. IV

LONDON

THE GRESHAM PUBLISHING COMPANY

34 SOUTHAMPTON STREET, STRAND

1903

BIOLOGY

UBRAPY

CONTENTS

HALF-VOL. IV

THE FOOD OF ANIMALS (Continued}

CHAPTER XXIII.— THE FOOD OF ANIMALS — OMNIVOROUS
ANNELIDS, SIPHON -WORMS, LAMP -SHELLS, MOSS -PO-
LYPES, AND WHEEL-ANIMALCULES

Page

BRISTLE-WORMS (Chaetopoda) — Lob-Worm, Tube- Worms (Serpula, Pomatoceros,

Spirorbis), Earth- Worms - - 257

SiPHON-WORMS (Gephyrea)— Common Siphon-Worm - 259

LAMP-SHELLS (Brachiopoda) - - - 260

MOSS-POLYPES (Polyzoa) - - - 261

WHEEL- ANIMALCULES (Rotifera)— Rose-coloured Rotifer, Crown Rotifer, Flower

Rotifer 261

CHAPTER XXIV. — THE FOOD OF ANIMALS — OMNIVOROUS
ECHINODERMS, SPONGES, AND ANIMALCULES— ANIMALS
WHICH FEED LIKE GREEN PLANTS

HEDGEHOG -SKINNED ANIMALS OR ECHINODERMS (Echinodermata) : Sea-
Urchins (Echinoidea) ; Sea-Cucumbers or Holothurians (Holothuroidea) ;
Crinoids (Crinoidea)— Feather-Stars and Sea- Lilies - - 264

SPONGES (Porifera) - 265

ANIMALCULES (Protozoa): Infusoria — Ciliata (Slipper Animalcule, Bell Animalcule);
Flagellata (Euglena, Collar Animalcules, Monads) ; Rhizopoda — Proteus Ani-
malcule or Amoeba, Foraminifera, Fungus-Animals (Mycetozoa) - 266

ANIMALS WHICH FEED LIKE GREEN PLANTS— Animals, Green Plants, and
Colourless Plants (Fungi, &c.) compared as regards Food and Feeding : Leaf-
Green or Chlorophyll — Animals which contain Chlorophyll — Green Planarian
Worm (Convoluta) ; Green Freshwater Polype, Coral- Polypes; Freshwater
Sponge; Green Animalcules (Berry Animalcule, Vol vox) 270

vi CONTENTS

ANIMAL DEFENCES

CHAPTER XXV. — ANIMAL DEFENCES — INTRODUCTORY —
BODILY CHARACTERISTICS PRODUCING INCONSPICU-
OUSNESS

Page

INTRODUCTORY — Different Kinds of Defence against Predaceous Forms. — I. PRE-
CAUTIONARY MEASURES:— (i) BODILY CHARACTERISTICS resulting in (a)
Inconspicuousness, (b] Conspicuousness : (2) MODE OF LIFE — (a) Feeding at
Favourable Times, and (b] Feeding in Favourable Places. — II. RESISTANCE: —
(A) PASSIVE DEFENCE by:— (i) BODILY CHARACTERISTICS such as (a) Un-
palatableness and Indigestibility, (b] Armour: (2) SPECIAL HABITS, e.g. Death-
feigning: (3) FECUNDITY :— (B} ACTIVE DEFENCE by means of (i) Ordinary
AGGRESSIVE WEAPONS, (2) Actively DEFENSIVE WEAPONS, (3) CO-OPERA-
TION:—III. RETREAT - - 275

INCONSPICUOUSNESS : — General Protective Resemblance ; Transparency ; Marine
Surface Animals; Snow Animals — Snowy Owl; Desert Animals — Camels,
Antelopes, Desert Foxes, Jerboas, Desert Larks, Desert Finches, Sand-Grouse,
Sand-Lizard, Thorn-tailed Lizards, Desert Monitor, Common Skink, Adder,
Horned Viper, Desert Insects; Reversed Shading; Flat Fishes - - - 277

Specialized General Resemblance— Corals and Sea-Snails, Corals and Brittle-Stars,

Sponges and Sea-Slugs ----------- 285

Protective General Resemblance in Eggs and Young — Plovers, Moths, Beetles - 285
Masking — Land-Snails, Sea-Snails, Crabs, Sea-Urchins, Sea-Anemones - 287

Variable General Resemblance: Colour-Change in Snow- Animals — Variable Hare,
American Hare, Stoat, Weasel, Ptarmigan; Colour-Change in Chameleons;
Colour-Change in Amphibians — Common Frog; Colour-Change in Fishes —
Plaice, Trout, Lumpsucker; Colour-change in Molluscs— Sea-Slugs; Colour-
Change in Crustaceans — ^Esop Prawn; Colour- Change in Insects — Peppered
Moth, Small Tortoise-shell Butterfly 289

Constant Special Protective Resemblance— Sloths, Pangolins; Coot, Moorhen,
Grebes, Young Plovers ; Australian Sea-Horses ; Sea-Slugs ; Stick-Caterpillars,
Leaf-Butterflies, Buff-tip Moth, Stick- and Leaf-Insects; Spiders - - - 294

Variable Special Protective Resemblance— Leaf-Butterflies, Caterpillars of Early

Thorn Moth - - 3<*>

CHAPTER XXVI.— ANIMAL DEFENCES— BODILY CHARAC-
TERISTICS PRODUCING CONSPICUOUSNESS

CONSPICUOUSNESS:— Genuine Warning and Spurious Warning (Mimicry) - - 301

Genuine Warning— Skunk; Coral Snakes, Cobras, Puff- Adders, Rattlesnakes;
Spotted Salamander, Nicaraguan Frog, Siamese Toad, Horned Toad; Weever-
Fish, Globe-Fishes; Ascidians, Acorn-headed Worms; Sea-Slugs; Wasps,
Hornets, Bees, Black-veined Brown, Magpie Moth, Conspicuous Caterpillars,
Lady-bird ; Spiders ; Bristle-Worms ; Planarian Worms ; Sea- Anemones, Corals,
Jelly-Fishes - 3°*

Spurious Warning (Mimicry)— Cuckoos, Orioles; American Snakes; Mimicking
Butterflies, White Ermine Moth, Clear-wing Moths, Caterpillars of Lobster-
Moth, Puss-Moth, and Hawk-Moths, Drone-Fly, Mimicking Beetles, Mimicking
Grasshoppers and Crickets, Praying Mantis, Mimicking Plant-Bugs ; Mimick-
ing Spiders - 3°9

CONTENTS vii

CHAPTER XXVIL— ANIMAL DEFENCES— SPECIAL
PRECAUTIONARY HABITS

Page

FEEDING AT FAVOURABLE TIMES: Nocturnal Animals — Night-Monkeys, Lemurs,
Bats, Elephant, Hoofed Mammals, Gnawing Mammals, Edentates, Marsupials,
Monotremes ; Owl-Parrot, Kiwi ; Geckos ; Amphibians ; Fishes ; Cockroaches,
Crickets, Moths, Fire-Flies; Marine Invertebrates - - 318

Diurnal Animals — Herbivorous Mammals, Species exhibiting Warning Colours and

Protective Resemblance - - - - - 323

FEEDING IN SUITABLE PLACES : Species exhibiting Protective Resemblance, Wide

Outlook, Proximity to Retreats - - 324

Arboreal Animals — Evolution of Climbing Forms - - - 325

Parachute Animals and Flying Animals— Their Evolution - 327

CHAPTER XXVIIL— ANIMAL DEFENCES-PASSIVE DEFENCE

UNPALATABLENESS AND INDIGESTIBILITY— Associated with Warning Coloration 332

ARMOURED ANIMALS— Armadilloes, Pangolins, Porcupines, Hedgehogs, Spiny
Ant-Eaters ; Feathers and Leg-Scales of Birds ; Armoured Reptiles ; Extinct
Armoured Amphibians, Ribs of Spanish Newt; Armoured Fishes; Shells of
Molluscs; Beetles, Weevils, Caddis- Worms ; Crabs, Rock- Lobster; Sea-Mouse,
Porcupine Worm, .Tube-dwelling Annelids; Skeletons of Moss-Polypes and
Lamp-Shells ; Plates and Spines of Sea-Urchins, Star-Fishes, &c. ; Armoured
Zoophytes— Hydroids, Organ-Pipe Coral, Sea-Anemones; Sponge Spicules;
Armoured Animalcules — Foraminifera, Radiolaria, &c. 333

ROLLING-UP HABIT— Armadilloes, Pangolins, Porcupines, Hedgehogs; Mail-Shells;

Trilobites, Wood-Lice 341

DEATH-FEIGNING HABIT — Dingo, Opossums, South American Fox; Tinamous,

Rails ; Lizards and Amphibians ; Spiders ; Beetles - - 342

FECUNDITY OF ILL-DEFENDED ANIMALS— Rabbit, Flesh-Fly; Bats as Check to

increase of Oak Procession-Moth 345

CHAPTER XXIX.— ANIMAL DEFENCES-ACTIVE DEFENCE

AGGRESSIVE WEAPONS IN DEFENCE - - - 348

ACTIVELY DEFENSIVE WEAPONS : Mammals — Teeth of Apes and Monkeys, Use
of Missiles by Baboons, Tusks of Walrus, Elephants, Rhinoceros, Wild Horses,
Teeth of Hippopotamus and Swine, Antlers and Horns of Ruminants, Claws
of Kangaroo, Stink-Glands of Skunk ; Birds and Reptiles — Legs of Ostrich and
Emeu, Rooks, Poisonous Lizards ; Poison-Spines of Fishes ; Poisoned Bite of
Cone-Shells, Stinging-Organs of Sea-Slugs, Shells of Tridacna; Stings and
Defensive Glands of Insects; Stink-Glands of Millipedes; Slime-Glands of
Peripatus; Bristles of Bristle- Worms ; Skin-Defences of Planarian Worms;
Poison-Spines of Sea-Urchins ; Stinging Organs of Zoophytes ; Irritant Rodlets
of Higher Animalcules ... - 348

CO-OPERATION AMONG SOCIAL ANIMALS — Wild Horses, &c. - 362

viii CONTENTS

CHAPTER XXX.— ANIMAL DEFENCES— RETREAT

Page

MAMMALS (Mammalia) — Monkeys and Baboons, Chamois, Antelopes, Rumination
as facilitating Retreat, Importance of Dwellings and Refuges with reference
to Retreat, Signalling Coloration - - 363

BIRDS (Aves) — African Ostrich, Rails, Extinction of Dodo, Flight as a means of

Retreat, Woodpeckers, Diving Birds - 367

REPTILES (Reptilia) — Brittle Tails of Lizards, Importance of Cylindrical Shape in

Snakes, &c. - - 370

AMPHIBIANS (Amphibia)— Wrestler Frog 371

INVERTEBRATES— Ink of Cuttle-Fishes, Detachable Foot-Region of some Land-
Snails, Springing Molluscs, Ants and Bombardier Beetles, Use of Suspensory
Threads (Spiders, Caterpillars, Slugs), Crustaceans, Sacrifice of Part of Body
in Annelids, Possible Origin of Segmentation - - 372

ANIMAL RESPIRATION— THE BREATH OF LIFE

CHAPTER XXXI.— ANIMAL RESPIRATION — GENERAL PRIN-
CIPLES—BREATHERS IN WATER AND BREATHERS IN AIR

GENERAL PRINCIPLES — Nature of and Necessity for Breathing or Respiration,

Breathing by General Surface, Special Breathing Organs 376

BREATHERS IN WATER AND BREATHERS IN AIR— Process essentially the same

in all cases, Relation between Plants and Animals as regards Breathing - - 378

CHAPTER XXXII.— ANIMAL RESPIRATION— VERTEBRATES
THAT BREATHE IN WATER

Nature and Development of Gill-Clefts, Their possible origin in remote ancestral

forms - - - - - - - - - - 381

FISHES (Pisces) AS BREATHERS IN WATER — Nature of Gills, Lampreys and Hags
(Cyclostomata), Sharks and Rays (Elasmobranchii), Sea-Cats (Holocephali),
Ordinary Bony Fishes (Teleostei) 383

PRIMITIVE VERTEBRATES (Protochordata) — Lancelet, Ascidians or Tunicates,

Acorn-headed Worms - - - 388

CHAPTER XXXIII.— ANIMAL RESPIRATION— NEMERTINES—
MOLLUSCS WHICH BREATHE IN WATER

NEMERTINE WORMS (NEMERTEA)— Some resemblance to Vertebrates - - - 391

MOLLUSCS (Mollusca) — Mail-Shells, &c. (Protomollusca); Head-Footed Molluscs
(Cephalopoda)— Cuttle-Fish, Pearly Nautilus; Snails and Slugs (Gastropoda) —
Ormer, Keyhole Limpet, Whelk and Purple Shell, Common Limpet, John
Knox's Limpet, Sea-Hare, Sea-Lemon, &c. ; Bivalve Molluscs (Lamellibranchia)
— Mussels, Cockles, Oysters, &c. - - 391

CHAPTER XXXIV.— ANIMAL RESPIRATION— JOINTED-LIMBED
ANIMALS WHICH BREATHE IN WATER

CRUSTACEANS (Crustacea) — Common Lobster, Freshwater Crayfish, Common
Prawn, Hermit -Crabs, Crabs, Mantis -Shrimps, Sand -Hoppers, Skeleton-

CONTENTS ix

Page

Shrimps, Sea-Slaters, Water Wood-Louse, Mud-Shrimps, Apus, Water-Fleas,
Mussel-Shrimps, Barnacles - - 400

KING-CRABS (Xiphosura) 406

CHAPTER XXXV.— ANIMAL RESPIRATION— LOWER INVERTE-
BRATES WHICH BREATHE IN WATER

SEGMENTED WORMS (Annelida) — Bristle- Worms (Choetopoda) — Scale- Worms,

Lugworm, Head-Gills of Tube-Dwellers; Leeches (Discophora) - - - 408

SIPHON-WORMS (Gephyrea) AND WHEEL-ANIMALCULES (Rotifera) - - 410

MOSS-POLYPES (Polyzoa) AND LAMP-SHELLS (Brachiopoda) - - - 410

HEDGEHOG- SKINNED ANIMALS OR ECHINODERMS (Echinodermata) — Influence
of the Skeleton on the Development of Breathing Organs, The Relation of the
Water-vascular System to Breathing - 41 1

ZOOPHYTES (Ccelenterata) — Breathing by External and Internal Surfaces, Special

Arrangement in some Sea-Anemones 416

SPONGES (Porifera) AND ANIMALCULES (Protozoa) - - - - - 418

CHAPTER XXXVI.— ANIMAL RESPIRATION— BACKBONED
ANIMALS WHICH BREATHE IN AIR

Nature and Development of Lungs, Aquatic Ancestry of Land Vertebrates - - 420
THE ORIGIN OF LUNGS — Swim-Bladder of Fishes — Bichir, Lung-Fishes - - 421
AMPHIBIANS (Amphibia) — Common Frog, Csecilians, Lungless Amphibians - - 422

REPTILES (Reptilia) — Increase in Complexity, Mechanism of Breathing, Crocodiles,

Snakes, Snake-like Lizards, Chameleons 424

BIRDS (Aves) — Structure of Lungs, Air-Sacs, Mechanism of Breathing - - - 426

MAMMALS (Mammalia) — Structure of Lungs, Mechanism of Breathing, Cetaceans,

Young Pouched Mammals - - - 427

CHAPTER XXXVII.— ANIMAL RESPIRATION -BACKBONELESS
ANIMALS WHICH BREATHE IN AIR

MOLLUSCS (Mollusca) — Lung-Snails and other Gastropods which breathe in air,

Garden Snail, Black Slug, Pond-Snail, Trumpet-Snail - ... 433

ARTHROPODS (Arthropoda) — Peripatus, Origin of Air-tubes ----- 434

CENTIPEDES AND MILLIPEDES (Myriapoda)— Shield-bearing Centipede, Imperfect

Development of Blood-System 435

INSECTS (Insecta) — Cockroach, Bees and Locusts; Aquatic Air-breathing Insects
—Great Water-Beetle, Whirligig Beetles, Great Black Water-Beetle, Pond-
Skaters, Water-Boatmen, Water-Scorpions, Rat-tailed Maggot, Common Gnat 437

ARACHNIDS (Arachnida) — Scorpions, Whip-Scorpions, Spiders, Mites - - - 442
CRUSTACEANS (Crustacea)— Land-Crabs, Wood- Lice - - 443

LAND NEMERTINES (Nemertea), EARTH-WORMS AND LAND-LEECHES (Annelida),

AND LAND PLANARIANS (Turbellaria) - - - - - - - - 444

x CONTENTS

CHAPTER XXXVIII.— ANIMAL RESPIRATION— AMPHIBIOUS
VERTEBRATES

Page

FISHES (Pisces)— Eel, Carp, Tench, Mud-Skippers, Bleak, Roach, Loach, Snake-
headed Fishes, Climbing Perch; Evolution of the Lungs of Backboned Animals,
Lung-Fishes 447

AMPHIBIANS (Amphibia) — Hell-Bender, Giant Salamander, Olm, Siren Salamander;

Common Frog - ... 456

CHAPTER XXXIX.— ANIMAL RESPIRATION— AMPHIBIOUS
INVERTEBRATES

MOLLUSCS (Mollusca) — Origin of Land-Snails and Slugs — Periwinkles, Apple-
Snails, &c. - 459

INSECTS (Insecta) — Net- Winged Insects (Neuroptera) — Stone-Flies, Dragon-Flies,
May-flies, Alder-Flies, Caddis-Flies ; Two-Winged Insects (Diptera) — Midges,
Sand-Midges - - 462

CRUSTACEANS (Crustacea) — Shore- Crabs, Robber- Crab - 469

LIST OF ILLUSTRATIONS

HALF-VOL. IV

COLOURED PLATES
THE CORAL SNAKE (Elaps Corallinus).

A Study by A. Fairfax Muckley Frontispiece.

PROTECTIVE ANIMAL COLORATION.

A Study by A. Fairfax Muckley

HEAD-FOOTED MOLLUSCS (Cephalopoda) (after Merculiano and Jatta).

A Study by A. Fairfax Muckley

312

392

BLACK-AND-WHITE ILLUSTRATIONS

Page

Group of Serpulae 258

Dissection of Front End of an Earth-Worm 259
Dissection of Siphon-Worm (Sipunculus]

(after Keferstein) 260

Moss-Polypes (after Kraepelin and Boas) - 261
Crown Rotifer (Stephanoceros] - - - 262
Vertical Section of Simple Sponge - - 265
A Proteus Animalcule (Amoeba) surround-
ing a slender Alga (after Rhumbler) - 269
Berry Animalcule (Hczmatococcus pluvialis)

in resting and motile stages - - - 273
Pallas's Sand- Grouse (Syrrhaptes para-

doxits] (from Brehm) .... 280
Eggs of Ringed Plover (sEgialitis hiati-

cula} (from a photograph by R. A. L.

Van Someren) 286

Crouching Young of Peewit ( Vanellus cris-

talus) (from a photograph by R. A. L.

Van Someren) 286

A Beetle (Lithinus nigrocristatus) which

resembles Lichen 287

Upper Side of a Xenophorus Shell (from

Chun) 288

Ptarmigan (Lagopus mutus), in winter

plumage 290

Australian Sea- Horse (Phyllopteryx eques} 296
Caterpillars of Brimstone Moth (Rumia

cratagata) in protective attitudes (after

Poulton) 297

Hinder End of Caterpillar of Brimstone

Moth (after Poulton) .... 297

Page

Indian Leaf Butterfly (Kallima inachis] - 298

American Skunk {Mephitis siiffbcans) - 302

Rattle of Rattlesnake . - - . 304

Horned Toad ( Ceratophrys ornata] - - 305
A Friar -Bird (Philemon Timorlaoensis)

mimicked by an Oriole ( Oriolus decipiens} 310
Caterpillars of the Lobster Moth (Stan-

ropusfagi) - - - - . 314

Heads of Nocturnal Animals - - - 319
Diagram of varieties of Reptilian Armour,

as seen in section (after Boas) - - 333
Nile Crocodile. Two scutes, covered by

horny epidermal plates - 333
Carapace and Plastron of a Tortoise (after

Boulenger) 334

Method of Growth in Gastropod Shells - 335

Growth of Bivalve Shell as seen in section 335

A spiny Sea- Snail (Murex) - - - 336

Opercula of various Gastropods - - 336

Caddis- Worm Tubes of various kinds - 337

The Northern Stone-Crab (Lithodes maia] 338
A Sea-Urchin (Echinus lividus\ showing

protective covering of spines - - 340
Part of Sea-Urchin Test, showing knobs

for attachment of- spines - - - 340
A species of South American Fox (Cants

azarce] 343

THE FALLOW DEER ( Cervtis dama) • 350

Horns of Gazelles 353

Poisonous Mexican Lizard (Heloderma

horridum} 355

XI 1

LIST OF ILLUSTRATIONS

Page

Poison Spines of Fishes (after Giinther) - 356
Sting of Bee (after Carlet) - - 358

Poison-Spine of a Sea-Urchin (Astheno-

soma urens) (after P. and F. Sarasin) - 361
Baboons retreating from Wild Dogs (from

Brehm) 364

THE CHAMOIS (Riipicapra Tragus) - 366
The Common Squirrel (Sciurus vulgaris) - 368
The Dodo (Didus ineptus] - - 369

Slug suspended by a Thread of hardened

Slime - - - 374

Front Part of Chick Embryo - - - 381
Californian Hag-Fish (Bdellosloma) - - 383
Diagram of Circulatory System in a Fish - 384
Diagram showing the Gill-Pouches of a

Lamprey (Petromyzon) - - - - 384
Circulatory and Breathing Organs of Hag

(Myxine glntinosa) (after J. Miiller) - 385
Horizontal Section through a Shark, show-
ing the Gill-Pouches) (after Gegenbaur) 386
The Sea-Cat (Chimara monstrosd) (after

Garman) 387

Horizontal Section through the Breathing

Organs of a Teleost (after Gegenbaur) - 388
Lancelet (Amphioxus lanceolatus) (after

Boveri) 389

Section through an Acorn-headed Worm

(Balanoglossus) (after Spengel) - - 390
Mail-Shell (Chiton] - ... 392

Dissection of a Cuttle-Fish (Sepia) to show

Gills (after Savigny) - ... 392
Ormer (Haliotis) dissected to show Gills - 394
Shells of various Sea-Snails (after Lang) - 394
Diagram of a Whelk (J5ucftnufft)(a.ftct Lang) 395
Gill-Cavities of John Knox's Limpet and

Common Limpet 396

Diagram of Sea- Hare (Aplysid) (after Lang) 396
Diagrammatic Cross Section of Hind-

gilled Snail 396

Sea- Lemon (Doris) (after Alder and Han-
cock) - .... 397
Freshwater Mussel (Anodonta) opened to

show Gills 398

Freshwater Mussel (Anodonta) imbedded

in mud 398

Gills of Lobster (Homarus vulgaris)- - 401
Gills of a Crab (after Gegenbaur) - - 404
Gills of Mantis-Shrimp and Sand-Hopper

(after Milne-Edwards and Sars) - - 404
Mud-Shrimp (Nebalia) (after Milne-Edwards) 405
A Mussel-Shrimp (Cypris), enlarged - 406
King-Crab (Limulus) (partly after Ray

Lankester) 406

Scale- Worm (PolynoS) (after Milne-Edwards) 408
Lugworm (Arenicola piscatoruni)^ showing

Gills 409

A Tube-Worm (Terebella), showing Gill-
plumes on Head 409

Page

Priapulus (after Ehlers) .... 4IO
Mouth-Area of a Sea-Urchin (Echinus

esculentus) (from Kiikenthal)- - - 412
Cross Section through Arm of Star-Fish

(from Kiikenthal) - - - - 413

Heart-Urchin (Spatangus purpureus) • 415
Dissection of Sea-Cucumber (after Ludwig,

simplified) 416

Diagrammatic Vertical Section of a Sea-
Anemone 417

Swim-Bladder of Bichir (Polypterus), dia-
grammatic - - - - - - 421

Sections through Lungs, showing Ingrowth
of Folds (after Boas) .... 424

Head of Crocodile to show Breathing

Arrangements (after Boas) - - - 425
Lungs of a Chameleon (after Wiedersheim) 425
Lungs and Air- Sacs of a Bird (after Heider) 426
Convoluted Windpipe of a Crane - - 427
Air- Passages of Lungs of Man - - - 428
Mouth, Nose, &c., of Man, in Section - 429
Lung of Land-Snail (Helix) (after Hatschek

and Cori) 433

Breathing Organs of Shield-bearing Centi-
pede (Scutigera) (after Haase) - - 436
Air-Tubes of Cockroach (Periplaneta ori-

entalis) (after Hatschek and Cori) - - 438
Dissection of Honey-Bee (Apis mellifica]

(after Leuckart) 439

The Drone-Fly (Eristalis tenax) and its

Larva, the Rat-tailed Maggot - - 441
Larva and Pupa of Common Gnat (Culex

pipiens), enlarged .... 442
Mygale (partly dissected) from below - 443
THE MUD-SKIPPER (Periophthalmus Koel-

reuleri) 448

Roach (Leuciscus rutilus) and Bleak

(Alburnus lucidus) - ... 449
Indian Snake-headed Fish (Ophiocephalus) 451
Climbing Perch (Anabas scandens) - - 452
Dissection of a Bitterling (Rhodeus amarus) 452
Roof of Gill-Cavity in a Species of Peri-
winkle (Littorina rudis) (after Pelseneer) 460
Apple-Snail (Ampullaria) (after Semper) - 461
A Stone - Fly (Pteronarcys} (after New-
port) - - - 464
Air-Tubes of Rectal Gills in Dragon-Fly

Nymph (after Oustalet) - - - 465
Nymph of Common May- Fly - - - 465
Hinder Part of Nymph of a May -Fly

(Cloeon dipterum) (after Zimmermann) - 465
Crustacean - like Nymph of a May -Fly

(Prosopistoma) (after Vayssiere) - - 466
Stages in Life-History of a Sand-Midge

(Simulia) (after Verdat) - . 468

Diagrammatic Cross Section through
Breathing Organs of Robber Crab
(Birgus latro) (after Semper) - - 469

CHAPTER XXIII

THE FOOD OF ANIMALS— OMNIVOROUS ANNELIDS,
SIPHON-WORMS, LAMP-SHELLS, MOSS-POLYPES,
AND WHEEL-ANIMALCULES

SEGMENTED WORMS (ANNELIDA)

The ANNELIDS include Leeches (Discophora) and Bristle-
Worms (Chaetopoda). The former are carnivorous, and have
already been dealt with (pp. 147-149), and the same is true for
many rapacious marine worms belonging to the latter group, as,
for example, the Sea-Centipede (Nereis] (pp. 146, 147).

OMNIVOROUS BRISTLE- WORMS. — These comprise marine, fresh-
water, and terrestrial forms. The marine species include a number
of worms which are entirely devoid of biting structures and burrow
in sand or mud, which they swallow in order to extract the nutri -
tive animal and vegetable matter present. The same habit has
already been described (p. 246) for the Acorn-headed Worm
(Balanoglossus), one of the lowest animals having any claim to
be considered a member of the Vertebrata. A good example
of marine worms which feed in this way is the Lob -Worm
(Arenicola) (see vol. i, p. 430), common on British shores be-
tween tide -marks where mud or mud- containing sand occurs.
This is a good-sized cylindrical form with broad front end, and
may attain a length of some 8 inches. It burrows in the sand
to a depth of about 2 feet, eating its way through, so to speak,
and from time to time comes to the surface for the purpose of
ejecting the sand which has passed through its body. This is
the origin of the little coils of mud or sand known as "worm-
castings " which are commonly seen upon the shore.

A considerable number of marine bristle-worms have given
up an active life and taken to live in tubes of various kinds,
these either consisting entirely of material exuded from the surface
of the body and hardened into horny or shelly substance, or of

VOL. II. 257 49

258

THE FOOD OF ANIMALS

foreign particles, such as sand-grains, glued together by a sticky
secretion. A typical example is afforded by the genus Serpula
(fig. 466), which makes and inhabits calcareous white tubes,
twisted masses of which are often found adhering to rocks,
oyster-shells, &c. It is clear that a tube-dweller like this has to
make special provision for breathing and feeding, and this is here

effected in an interesting manner. On
watching a living Serpula placed in a
vessel of sea-water the head end will
soon be seen to protrude. First of all
a sort of conical stopper (operculum),
which closes and protects the mouth of
the tube, is pushed out, and then follow
two brightly -coloured plume -like out-
growths from the head. These are
covered with cilia, which set up currents
in the surrounding water, as a result of
which breathing is provided for, while
at the same time a constant stream of
edible particles is directed into the mouth.
A common and, when expanded, very
attractive - looking worm (Pomatoceros
triqueter) that abounds on the British
coast, lives in a small wavy tube attached to a stone or other
firm body. One end of the tube tapers to a point, while the
aperture is overhung by a sharp projecting spine from which a
prominent ridge runs backwards. Equally common is a still
smaller tube-worm (Spirorbis], which inhabits a calcareous tube
coiled into a flat spiral, and adhering to brown sea-weeds or other
suitable objects. Other tube-dwellers will be considered when
animal habitations are described.

Great interest attaches to the widely -distributed group of
Earth- Worms, the habits of which, so far as British species are
concerned, were first studied in detail by Darwin, the results
being embodied in his classic work on the subject.

A number of species are native to Britain (a common sort
being Liimbricus herculeus), all of which live in much the same
way. Examination of one of them shows the complete absence
of jaws, and the food chiefly consists of earth which is constantly
swallowed for the sake of the organic matter it contains. An

Fig. 466. — Group of Serpulae. Two indi-
viduals are projecting from their tubes

OMNIVOROUS ANNELIDS, &c.

259

BUCCAL POUCH

earthworm practically eats its way through the ground, and the
earth which has passed through its body is from time to time
deposited on the surface in the form of "castings", much as in
the case of the Lob- Worm (p. 257). By carefully weighing the
castings deposited on a known area Darwin came to the conclusion
that in many parts of England 10 tons
per acre of soil annually pass through the
bodies of these creatures. Earth-worms
also devour small pieces of vegetable or
animal matter which come in their way.
There are several points of interest in the
structure of the digestive organs (fig. 467).
The mouth opens into a small pouch which
can be protruded to the exterior to aid in
the taking in of food, and this followed by
a muscular pharynx which appears to exert
a sort of sucking action. Next comes a
slender gullet into which open glands
secreting particles of carbonate of lime
that perhaps help to neutralize organic
acids present in the soil, and the posterior
end of this dilates into a rounded crop in
which food is temporarily stored, and which again communicates
with a rounded muscular gizzard, enclosing small stones, much as
in a bird. These help to grind up the food, and thus make up for
the absence of jaws. The rest of the digestive tube consists of
the long thin-walled intestine, in which the process of digestion
is completed. A further fact of interest, observed by Darwin, is
that a digestive fluid is poured from the mouth upon bits of
vegetable matter, the preparation of which, therefore, begins out-
side of the body altogether.

SIPHON-WORMS (GEPHYREA)

These are jawless marine worms, some of which are om-
nivorous sand-swallowers devoid of bristles. A good example
is the Common Siphon - Worm (Sipunculus) (see vol. i, pp.
433, 434), found burrowing in sand on many parts of the British
and other coasts. The front part of the body is narrow, and when
fully expanded is seen to end in a horse-shoe-shaped circlet of
short tentacles, within which is the mouth. The greater part of

Fig. 467. — Dissection from above of
front end of an Earth-Worm

260

THE FOOD OF ANIMALS

the gut consists of a long very thin-walled intestine, which pursues
a spiral course to the hind end of the body, and then twines back

upon itself to terminate upon the upper
side of the body not far from the front
end (see fig. 468). It is always full
of sand, and siphon -worms of the
sort described play much the same part
in regard to the sand of the sea that
earth-worms do as regards the earth
covering the land. Shipley (in The Cam-
bridge Natural History) makes the fol-
lowing remarks on this head : — ' The
rate at which the sand passes through
the body of Sipuncuhis is unfortunately
unknown, but that at any one moment
a considerable quantity is contained in
the intestine is shown by the fact that
the average weight of five specimens of
S. nudus [a large species] from Naples,
taken at random, was 19.08 grms., whilst
the average weight of sand washed out
of their alimentary canal was 10.03
grms ".

Three groups of specialized worms
now claim our attention, i.e. Lamp-
Shells (BRACHIOPODA), Moss- Polypes
(POLYZOA), and Wheel-Animalcules (Ro-
TIFERA). All the animals included in
these groups are omnivorous, feeding
t, Tentacles; mm and m'm'm'm', muscles upon the minute organisms and nutritive

which pull in front end of body, cut through; • 1 i 1 i i M«

gi. guiiet -,int. intestine;/.*. intestinal aper- particles brought to them by ciliary cur-

ture; ex. ex. excretory tubes; br. brain; „„„,.„ /0<a<a rM^ o A t ^> A A\
nc, nerve cord. rGntS VSGe PP' 243» 244)'

LAMP-SHELLS (BRACHIOPODA)

The animals here included were once confounded with the
bivalve Molluscs (p. 248), chiefly because they are enclosed in a
shell composed of two pieces. A typical Lamp-Shell (see vol. i,
p. 439) is attached to some firm body, either by the substance of
one shell or by means of a sort of stalk projecting at one end.
As has already been exemplified in other groups, sedentary or

OMNIVOROUS ANNELIDS, &c.

261

fixed forms commonly get their living by setting up currents in
the surrounding water. In this case the current-producing organs
are two often complicated "arms", fringed outgrowths from the
mouth-region richly covered with cilia, the movement of which
produces food-bearing streams of sea-water, which are conducted
along grooves to the mouth. All Brachiopods are marine, and
at the present time they exist in greatly-diminished numbers,
though in older geological epochs they were dominant forms of
marine life.

MOSS -POLYPES (POLYZOA)

The Moss- POLYPES are small animals, nearly always fixed,
and in most cases living in colonies produced by budding (vol. i,
pp. 436-438). Most of them are marine, but several kinds are
inhabitants of fresh water. A great many species are found

MOUTH
GULLET

Fig. 469. — Moss-Polypes (enlarged)

A, Small colony of Lophopus crystallines, showing some individuals fully extended, and
others in different states of retraction. B, Diagram of a single individual of Plumatella, cut
through centre of body.

attached to sea- weeds, &c., along the British coasts, and there
are also some native species in our streams and lakes. The
projecting end of each individual bears both openings of the
U-shaped gut, and also a crown of ciliated tentacles (lophophore)
which has been compared to the ''arms" of the preceding group,
and which has at any rate the same chief function, the setting up
of food-bearing currents in the surrounding water (fig. 469).

WHEEL-ANIMALCULES (RoxiFERA)

These minute creatures mostly live in fresh water, including
puddles, roof-gutters, and the moisture saturating damp vege-

262

THE FOOD OF ANIMALS

tation, especially mosses, but some are to be found in brackish
water or in the sea. They present a great variety in form and
habit; the majority are free-swimming, but others live in tubes
constructed by themselves, and others again are parasitic. A
typical free - living form has elsewhere been briefly described
(vol. i, pp. 434, 435). In by far the greater number food is brought
to the mouth by ciliary action, set up by variously-arranged cilia
placed on the front end of the body and sometimes situated on.
special lobes. The name "wheel-animalcule" was given by early
observers, who studied species in which there are two circlets of
cilia placed on adjacent projections. The successive movement
of the cilia on these projections produces an optical illusion, and
suggests the movement of a wheel. Thus, Baker, writing about
1744 to the then President of the Royal Society concerning his
observations on the form already described, the Rose-coloured
Rotifer (Philodina roseola] (see vol. i, p. 434), gives the following
account, which, though it mistakes the nature of the " wheels",
which is not surprising, clearly grasps their use
in feeding: — " If the water standing in gutters
of lead, or the slimy sediment it leaves behind,
has anything of a red colour, one may be almost
certain of finding them therein, and, if in sum-
mer, when all the water is dried away, and
nothing but dust remains, that dust appears
red, or of a dark-brown, one shall seldom fail,
on putting it into water, to discover multitudes
of minute reddish globules, which are indeed
the animals, and will soon change their appear-
ance in the manner just now mentioned. . . .

"A couple of circular bodies, armed with
small teeth like those of the balance-wheel of a
watch, appear projecting forwards beyond the
head, and extending sideways somewhat wider
than the diameter thereof. They have very
much the similitude of wheels, and seem to turn
round with a considerable degree of velocity, by which means a
pretty rapid current of water is brought from a great distance to
the very mouth of the creature, who is thereby supplied with many
little animalcules and various particles of matter that the waters
are furnished with."

Fig. 470. — Crown Rotifer
[Stephanoceros], enlarged

OMNIVOROUS ANNELIDS, &c. 263

All wheel-animalcules, however, are not entirely limited, as
regards food -supply, to what is brought by ciliary currents.
Some of them (fig. 470), such as the beautiful Crown Rotifer
(Stephanoceros], possess a number of pointed projections from
the head - region, which are actively used to surround small
animals or other food substances, and their efficiency may be
increased by the presence of slender bristles, as in the Flower
Rotifer (Floscidarid). The mouth of a wheel -animalcule leads
into a funnel-shaped pharynx lined with cilia, and this again is
continuous with a powerful muscular gizzard (mastax) imbedded
in the walls of which are hard jaw-like pieces used for chewing.
In many cases these jaws can be protruded from the mouth and
employed for seizing food, even, it may be, for attacking other
rotifers with a view of adding them to the bill of fare.

CHAPTER XXIV

THE FOOD OF ANIMALS— OMNIVOROUS ECHINODERMS,
SPONGES, AND ANIMALCULES— ANIMALS WHICH
FEED LIKE GREEN PLANTS

ECHINODERMS (ECHINODERMATA)

The large and characteristic phylum of ECHINODERMS (p. 153)
embraces Star -Fishes, Sea -Urchins, Sea -Cucumbers (Holothu-
rians], and Crinoids (Sea- Lilies and Feather-Stars). The first
of these are essentially carnivorous, and have already been dealt
with (p. 153), but the remainder, being largely omnivorous,
claim some attention here. They exemplify feeding by sand-
swallowing and the action of ciliary currents, both being common
modes of obtaining nutriment, as has already been shown.

SEA-URCHINS (see vol. i, p. 456) are spheroidal or flattened
forms, covered with movable spines, and with the mouth placed
on the under surface. Five pointed jaws, which grow continu-
ously, like the front teeth of a rabbit, are often but not always
present. The thin coiled intestine is, in many species, found
on dissection to be full of sand, from which the organic matter is
extracted just as in a Lob- Worm (p. 257) or Siphon- Worm

(P- 259)-

SEA-CUCUMBERS (Holothurians] (see vol. i, p. 462) are worm-
like animals with thick leathery skins. The mouth is placed
at the front end, and is surrounded by a circlet of tentacles, the
shape of which varies a great deal in different species. They
are generally used for shovelling sand or mud into the mouth,
but in some cases are long and branched, so that when fully
extended they can be employed as a sort of net in which food
floating in the surrounding water gets entangled.

The commonest existing CRINOID is the Feather-Star (Com-
atula] (see vol. i, p. 460), which consists of a central disc, from
which ten feathery arms radiate, the plume-like appearance of

264

OMNIVOROUS ECHINODERMS, &c.

265

these being due to a series of short branches (pinnules) with
which each of them is beset on either edge. The Feather- Star
is commonly found moored to some firm object by a circlet of
filaments attached to the back of the central disc. The mouth
is situated in the centre of the body, and from it ciliated food-
grooves run along the arms, sending branches along the pinnules.
The currents set up by the action of the cilia flow along the
grooves to the mouth, into which they carry all sorts of minute
organisms and other particles serving as food.

The other members of the Crinoidea are Sea- Lilies (see vol.
i, p. 460), which live in the deep sea, and may be compared to
Feather -Stars attached to various objects by long stalks. Their
way of feeding is precisely the same.

Passing over the members of the phylum COZLENTERATA,
which in typical cases are actively carnivorous (see p. 155), we
come to the two low-
est groups in the
animal kingdom, i.e.
SPONGES and PROTO-
ZOA, the species in-
c4uded in which are
mostly or largely

omnivorous.

SPONGES (PORIFERA)

SPONGES are either
simple or colonial
animals of sluggish
habit, which live at-
tached to stones or
other firm objects.
The structure of a
simple Sponge has
been de-

F'S' 47I-~Vertical Section of Simple Sponge (enlarged and diagrammatic)

ec, External body-layer; m, middle body-layer, with spicules, s; en, inter-

(VOI. 1 D. 4.84.) nal body-layer, made up of flagellated cells and extending into the canals (a/),
. ill which pierce the side-wall of the body, opening into the central cavity, g,

-and. it need Only DC that also communicates with the exterior by a large aperture, a. The arrows
•11 i i indicate the course taken by the currents of water which traverse the body.

stated here that the

body is vase-shaped, with its walls perforated by numerous holes.

The lining of the vase is composed of a layer of remarkable

266 THE FOOD OF ANIMALS

collar-cells, from each of which projects a thread of living sub-
stance (protoplasm) which constantly describes whip-like move-
ments, and has therefore been termed a flagellum (L. ftagellum,
a whip). By the combined action of all the flagella, currents are
produced in the surrounding water, which flow through the holes
in the body-wall and then to the exterior through the mouth of
the vase (fig. 471). These currents bring with them food of mixed
vegetable and animal nature, as in so many other cases where
this mode of getting a living is adopted. Colonial Sponges
may assume all sorts of shapes, and their structure is often very
complicated, but in all cases the food is obtained as in the simple
case described.

ANIMALCULES (PROTOZOA) (see vol. i, fig. 301)

We have here a host of microscopic or minute forms, the chief
omnivorous species belonging to the two important groups of
Infusoria and Rhizopoda.

INFUSORIA. — These include forms which are more or less
covered with the short vibratile threads known as cilia, and also
others provided with one, two, or a small number of the longer
protoplasmic threads termed flagella. The former set of species
constitute the Ciliata, which are either simple or colonial, free-
swimming or fixed, and the same variety in character and habit
is presented by the flagellate species, embraced in the Flagellata.

Among the Ciliata (see vol. i, p. 489) the free -swimming
Slipper Animalcule (Paramcecium) and the fixed Bell Animalcule
(Vorticella) will serve as examples. The Slipper Animalcule is
a small active creature of oval shape, which is just visible to
the naked eye. It moves actively about by means of the cilia
which cover its body, and on the under side is a ciliated de-
pression which conducts food particles to the mouth and thence
to the semifluid interior of the body. With each batch of
solids a small amount of water passes in, and the food-containing
globule is termed a " food-vacuole ". The food-vacuoles are
moved slowly round the inside of the animal, digestion and ab-
sorption going on meanwhile, and the undigested remnants are
cast out at a point not far from the mouth. The Bell Animalcule,
when fully expanded, is shaped like a blunt cone attached by a
stalk at the narrow end, while the broad part bears a wreath
of cilia arranged in a short spiral. These produce a sort of

OMNIVOROUS ECHINODERMS, &c. 267

whirlpool by which food is carried down into a ciliated funnel
at the bottom of which the mouth is placed, and through this
the nutritious particles, together with water, are conducted down
a short gullet into the soft interior of the body. The remaining
details are much as in the Slipper Animalcule.

The Flagellata (see vol. i, p. 489) include a host of minute
creatures, many of which can only be studied by means of a
powerful microscope. Suitable examples are Euglena, Collar
Animalcules, and Monads.

Euglena. — Stagnant pools, or puddles of fresh water, and
also the moisture accumulating in roof-gutters, are often of a
greenish colour, as a result of the presence of immense numbers
of Euglenae. Each individual is somewhat sausage-shaped, but
the writhing, worm-like movements which are constantly going
on alter its appearance from time to time. A long, quivering
flagellum projects from one end of the body, and at its base is
a minute mouth from which a short, funnel-like gullet projects
into the soft interior substance of the body. The flagellum
sets up movements in the surrounding water by which minute
food particles of various kinds are drawn into the mouth.
Euglena is green in colour, owing to the presence of green
pigment identical with that found in the leaves of ordinary plants.
There is also a red " eye-spot " near the front end.

Collar Animalcules are simple or colonial forms, in which
each individual is comparable to a collar-cell of a sponge
placed on a stalk and leading an independent existence. As in
Euglena, small particles of food are drawn to the neighbour-
hood of the animal by the movement of the flagellum. Some of
these stick to the outside of the collar and are carried down to
a soft spot equivalent to a mouth.

Monads are excessively minute Flagellates which swarm in
putrefying infusions. A typical species is the Springing Monad
(Heteromita), in which the body is pear-shaped and bears two
long flagella. One of these is directed forwards, and serves as
the active agent of locomotion, while the other is trailed behind.
This is one among very numerous cases in the Protozoa where
the food does not consist of solid particles taken in through a
mouth. In fact the Springing Monad would seem to be devoid
of a mouth, and feeds upon the nutritious substances dissolved in
the infusion where it lives, which are able to diffuse into the

268 THE FOOD OF ANIMALS

interior of the body through the delicate membrane by which it is
invested.

RHIZOPODS (see vol. i, p. 489). — These are animalcules of
lower grade than the preceding group, for the body is not
covered by a membrane conferring a definite shape, and is de-
void of the specialized threads (cilia and flagella) which, as we
have just seen, play such an important part in the feeding of
higher Protozoa. The simplest and at the same time most in-
structive example is the Proteus Animalcule (Amoeba), a micro-
scopic creature found in all sorts of places, especially on mud and
water-plants in ponds and ditches. Its body is a mere particle
of living substance (protoplasm), semifluid in consistency, and
continually altering its shape when the animal is in an active
healthy state, a circumstance which has suggested both the
common and scientific names. On watching a living amoeba,
which has been placed in a drop of water under the microscope,
its body will be observed to flow out from time to time into
bluntish lobes, which, however, are in no sense permanent struc-
tures, for they are sooner or later drawn back so as to form
part of the general body. Each such lobe is somewhat inappro-
priately called a cc false-foot " (pseudopod), and its use is to help
in locomotion and the taking in of food. Amoeba possesses
neither mouth nor internal digestive cavity, for in the absence of
a firm bounding membrane nutritive particles can be taken in
at any point, being engulfed by the pseudopods. The body,
so to speak, flows round the food, which is of varied nature,
consisting of microscopic plants and animals, together with
organic particles of different kinds. With these a good deal of
water is taken into the body, and food-containing globules (food-
vacuoles) result, much as in a Slipper Animalcule or Bell Animal-
cule (see vol. i, p. 492). These move round in the interior of the
animal and are gradually digested, the innutritious or undigested
parts being cast out of the body wherever most convenient. It
might almost be said that an amceba flows away from the re-
mains of its meals. The Proteus Animalcule has been observed
feeding upon certain lowly green plants (algtz] much longer than
itself, and the tactics pursued in order to get these entirely within
the body are extremely interesting (fig. 472). There is, of course,
no difficulty in flowing round one end of such a plant, and the
next move is for a pseudopod to be pushed along it, and then

OMNIVOROUS ECHINODERMS, &c.

269

bent back so as to make a kink or bend in the filament. By
a continuance of this sort of manoeuvring the animalcule often
succeeds in coiling up the alga into a compact mass of con-
venient shape, adapted for easy digestion. In briefly dealing
with human anatomy and physiology it was pointed out (vol. i,
p. 39) that the circulatory fluids of the body, blood and lymph,
contain innumerable microscopic bodies, the white corpuscles,
which progress by creeping movements, in the same way as just

Fig. 472. — A Proteus Animalcule (Amoeba} surrounding a slender alga. The numbers fi-8)
indicate the successive stages in the process. Much enlarged.

described for the Proteus Animalcule. Nor does the resemblance
between the two stop here, for the white corpuscles feed by taking
in particles bodily, a fact of great importance, and one which has
caused them to receive the name of "eating-cells " (phagocytes —
Gk. phagein, to eat; cytos, a small box, hence a cell). The white
corpuscles, indeed, play a very important part in most if not all
groups higher in the scale than Protozoa, for they perform the
functions of scavengers and police. In human beings, for example,
they attack and devour disease -germs which have made their way
into the system, and the upshot of many cases of infectious dis-
ease depends upon the result of a vigorous contest between cor-
puscles on the one hand and germs on the other.

Some very interesting freshwater Rhizopods are practically
amoebae provided with shells, and a shell-bearing group some-
what more distantly related is that of the Foraminifera, members
of which abound in the sea, and are also found to a less extent

270 THE FOOD OF ANIMALS

in the waters of the land. In these animalcules the pseudopods
are exceedingly slender, and united together into a viscid net-work
by which the food is entangled. The same device is present in
the shelless Fungus- Animals (Mycetozoa), of which one common
kind (^£thalium) is found in the form of good-sized branching
yellow flakes (" flowers of tan") creeping upon heaps of spent
tan. Each flake has been constituted by the fusion of a large
number of minute organisms resembling amoebae, and is there-
fore in reality of a compound nature.

ANIMALS WHICH FEED LIKE GREEN PLANTS

Some account has now been given of both food and way of
feeding in typical carnivorous, vegetarian, and omnivorous animals.
To complete and round off this part of the subject some notice is
necessary of certain animals, belonging to various groups, which
subsist partly or entirely after the manner of green plants. An
average ANIMAL feeds upon very complex food, part of which is
in the form of solid particles, for the reception of which an in-
ternal digestive cavity is provided. In all the forms so far
described the food is complex as regards its chemical nature,
though it may be entirely liquid, as in Tape -Worms, &c., and
examples have also just been given of animalcules devoid of any
digestive cavity. PLANTS, on the other hand, subsist entirely
upon gaseous and liquid food, though to this there are partial
exceptions. And though certain colourless plants, of which fungi
(e.g. mushrooms, toadstools, mildews, moulds) are familiar ex-
amples, so far approximate to animals that they live upon more
or less complex organic substances, yet typical green plants
differ markedly from average animals in that their food is
simple as regards chemical composition. The power such plants
possess of building up complex living substance (protoplasm)
from water, carbonic acid gas, and dissolved mineral substances,
depends upon the presence of the characteristic pigment called
leaf-green (chlorophyll). Examination of one of the thin leaves
from a moss-plant will show that it is made up of cells (vol. i,
p. 469) bounded by membranes (cell -walls) and containing
living substance (protoplasm). Imbedded in the protoplasm are
a number of rounded granules of bright-green colour. These
are chlorophyll-bodies > and each of them is in reality a specialized
bit of protoplasm through which leaf-green is diffused. Similar

OMNIVOROUS ECHINODERMS, &c. 271

bodies are present in the green parts, especially the leaves, of
all the plants which make up the conspicuous part of vegetation,
though in some lowly forms there may be chlorophyll-bodies of
very different shape. That, however, is an unimportant detail,
the essential fact being that leaf-green in some way enables the
living substance which it permeates to build up the simple
chemical compounds already enumerated into complex organic
substance. In an ordinary land-plant the requisite carbonic acid
gas (CO2) is supplied by the air, while the water and mineral
matters are taken up from the soil. In a word, green plants
bridge over, as it were, the gap between the non-living mineral
kingdom and living organisms. The upbuilding process asso-
ciated with the presence of chlorophyll is dependent upon light,
and this pigment, in some way not clearly understood, enables
the energy of the sun's rays to be used for the purpose.

The nutrition of green plants has been mentioned here because
there are some animals in which chlorophyll is present, and which
therefore are partly, or in some cases it would seem entirely, able to
subsist upon carbonic acid gas, water, and mineral salts. A very
interesting example is afforded by a small marine flat-worm be-
longing to the group of Planarian Worms (Turbellaria) (see vol. i,
pp. 445-447). This creature (Convoluta Roscoffensis) is found
in large numbers floating in the shore-pools at Roscoff, on the
French coast, and is coloured green by the presence of numerous
chlorophyll-bodies, which enable it to live entirely after the manner
of a green plant. The Convoluta belongs to a carnivorous group,
and is no doubt descended from forms which depended upon
animal food. A gradual change of habit has, however, taken
place, and this is associated with corresponding modification of
structure. It is, however, doubtful whether the chlorophyll-bodies
are really an actual part of their possessors. Some authorities
regard them as resulting from the specialization of microscopic
green plants (algae), the ancestors of which acquired the curious
habit of living inside an animal.

Well-known instances of the presence of chlorophyll are found
in the phylum of ZOOPHYTES (Ccelenterata). A case in point
is that of the Green Freshwater Polype (Hydra viridis) com-
monly found adhering to water- weeds in ponds and streams.
Hydra has already been described in some detail (vol. i, pp.
465-473), and it need only be remarked here that the body is

c72 THE FOOD OF ANIMALS

a tube closed at one end, by which attachment is effected, and
open at the other, the aperture being the mouth, around which
a number of slender tentacles, used for catching food, are arranged
in a radiating manner. The animal is indeed little more than a
living stomach. The body-wall consists of two layers, an outer
(ectoderm) and an inner (endoderm), with which latter we are
here concerned. For imbedded in this layer are a large number
of chlorophyll-bodies, which almost certainly enable the animal to
live partly upon simple inorganic substances, though the chief
food consists of small animals captured by means of the tentacles.
Here again it is considered by some that the green granules are
really minute plants, but this remains to be proved. Saville Kent
(in The Great Barrier Reef of Australia) expresses the opinion
that many kinds of coral-animals live entirely in a plant-like way,
an opinion founded on the absence of animal substances in the
digestive cavities of specimens examined by him. The matter
needs careful investigation, and it is quite possible that these
creatures may turn out to subsist in the duplex manner supposed
to be characteristic of the Green Hydra. The fact that coral-
animals are often of the most brilliant colours other than green
is no difficulty, for even in some green plants chlorophyll is dis-
guised by the presence of other pigments, of which a good
instance is that of the common brown sea- weeds. If a bit of
such a plant be immersed in spirit for a short time it will turn
green, as the brown colouring matter is quickly dissolved out;
and perhaps chlorophyll is not the only pigment which possesses
the remarkable properties above described.

Passing over the case of the Freshwater Sponge (Spongilti)*
which is of a green colour owing to the presence of chlorophyll,
we come to certain animalcules (Protozoa) in which this pigment
is present to a greater or less extent. Some of these are un-
doubted animals, and Euglena, which has been described above
(p. 267), is a good example of such cases. It partly feeds by-
taking in solid particles through a minute mouth, and also con-
tains chlorophyll-bodies, which no doubt enable it to utilize the
simple substances upon which green plants entirely subsist. From
a case like this we can pass to others where it is difficult, if not
impossible, to say whether we are dealing with plants or animals.
A familiar instance is afforded by a microscopic form which often
abounds in puddles, water-butts, &c., imparting a greenish tinge

OMNIVOROUS ECHINODERMS, &c. 273

to the water. It has no popular name, but to avoid using the
somewhat long-winded scientific one (H&matococcus phivialis]
(fig. 473), we may perhaps call it the Berry Animalcule. It is
to be found both in a "resting" and a "motile" stage. In the
former condition it appears as a minute sphere, either entirely
green, or more or less red from the presence of a second pigment
in addition to the chlorophyll. The sphere owes its definite form
to the presence of a delicate bounding membrane (cell-wall) com-
posed of a substance (cellulose) characteristic of plants, though not
entirely limited to them. Cellu-
lose is closely related to starch,
and ordinary cotton is a very
pure form of it. There is no
trace of mouth or digestive cavity,
and the food consists merely of
carbonic acid gas, water, and
simple mineral salts, and if our
knowledge of the Berry Animal-
cule Were limited tO this resting- Fig. 473.— Berry Animalcule (Hamatococcus plu-vi-
•i •i1 i-i nil alis}, greatly enlarged. A, Resting stage; B, motile

stage we should undoubtedly look Stage: ,.«,. Ceii-waii; ». nucleus; ji. flageiia.
upon it as a low green plant.

But we also find it in a " motile " condition, when it is capable
of executing active swimming movements. Under these circum-
stances the living substance of the body is pear-shaped, with a
couple of whip-like flageiia projecting from the clear narrow end,
and effecting active locomotion by means of their lashing move-
ments. In this case the cell-wall may either be entirely absent
or it may be seen investing the body some little distance from the
pear-shaped body -mass, and pierced by the projecting flageiia.
As before, no special digestive organs are present, while the
food and way of feeding are entirely plant-like. Considered by
itself, we should be justified in considering the Berry Animalcule
as a plant, and its powers of active movement in one stage of the
life-history is no particular bar, for many lower green plants (algae)
about whose nature there is no question propagate by means of
minute actively -moving "zoospores" of fairly similar character.
Most botanists accordingly claim this organism as a plant, and
it will be found described as such in standard text-books of
botany. Many zoologists, however, look upon the Berry Ani-
malcule as an animal which feeds like a plant, basing their

VOL. II. 50

276 ANIMAL DEFENCES

such as "precautionary measures" and "passive defence", the
defence is of purely unconscious kind, and has been determined
by evolutionary factors working irrespective of the will or know-
ledge of the animal. Even when an animal actively defends
itself against attack it does not follow that it has any clear idea of
what the aggressor is after, though it must be admitted that the
latter has more clearly-cut ideas on the subject.

The following scheme will perhaps serve to illustrate the
main principles of animal defence in the limited sense indicated
above, and it will be followed by detailed description of illustrative
instances, though space forbids anything like a complete review
of the animal kingdom under each heading.

I. PRECAUTIONARY MEASURES

Under this heading will be grouped those various defensive
measures which tend to obviate attack altogether, and these fall
pretty naturally under (i) Bodily Characteristics, and (2) Mode
of Life or Habits.

(1) BODILY CHARACTERISTICS. — These may be considered in
so far as they result in (a) Inconspicuousness, and (fr) Conspicu-
ousness. The first is obvious and the other will be clear after
explanation.

(2) MODE OF LIFE. — Defence may be promoted by (a) Feeding
at favourable times, (b) Feeding in favourable places.

Supposing, however, that attacks are actually delivered, their
intention may be frustrated by means falling under a second large
heading.

II. RESISTANCE

This may take the form of either (A) Passive Defence, or
else (B) Active Defence.

(A) PASSIVE DEFENCE. — This may be effected by (i) Bodily
Characteristics, or by (2) Special Habits, and not infrequently
both contribute to the efficiency of the resistance offered. There
is also (3) Fecundity as a defensive measure.

(i) BODILY CHARACTERISTICS. — Of these perhaps the most
important are — (a) Unpalatableness and Indigestibility,
resulting from various peculiarities ; (b) Possession of

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 277

Armour, as purely defensive arrangements may be broadly

termed.
(2} SPIX IAL HABITS, such, e.g., as the Death-feigning Instinct,

Rolling-up Instinct, &c.
(3) FECUNDITY as a means of defending the species.

(B) ACTIVE DEFENCE. — This employs the method of counter-
attack, and includes, among other things — (i) Use of ordinary
AGGRI ssiYi; WEAPONS; (2) Use of actively DEFENSIVE WEAPONS,
such as stings, stink-glands, &c. ; (3) CO-OPERATION for defensive
purposes between members of a community.

An attacked animal, however, may be unfitted to offer either
active or passive defence, or may reserve its resistance, and in
both these cases commonly endeavours to effect its escape. This
may be considered under a third main heading, i.e.—

III. RETREAT

It is scarcely necessary to tabulate the various ways in which
this is effected by different forms. Many animals are possessed
of great running powers, others can burrow rapidly, and so on.
The existence of dwellings or refuges is often a great help to
retreat, which may also be assisted by various special devices.

It must, of course, be understood that there are no sharp
lines of separation between animals which employ different modes
of defence, for some species may fall under more than one heading,
and even those which in the first instance attempt to retreat may
nevertheless offer a determined resistance should they be over-
taken by their foes, e.g. deer and poisonous snakes.

I. PRECAUTIONARY MEASURES

(i) BODILY CHARACTERISTICS.—

(a) Arrangements which bring about INCONSPICUOUSNESS. A
particular kind of animal may be possessed of peculiarities of
form or structure, or both, in virtue of which it harmonizes so
well with the surroundings that detection by an enemy becomes
a difficult matter. But to reap the full benefit of this a form so
protected must remain perfectly motionless, as the least movement
may at once destroy the illusion. And since movement is abso-
lutely necessary to the great majority of animals, the protective

276 ANIMAL DEFENCES

such as " precautionary measures" and "passive defence", the
defence is of purely unconscious kind, and has been determined
by evolutionary factors working irrespective of the will or know-
ledge of the animal. Even when an animal actively defends
itself against attack it does not follow that it has any clear idea of
what the aggressor is after, though it must be admitted that the
latter has more clearly-cut ideas on the subject.

I. PRECAUTIONARY MEASURES

Under this heading will be grouped those various defensive
measures wrhich tend to obviate attack altogether, and these fall
pretty naturally under (i) Bodily Characteristics, and (2) Mode
of Life or Habits.

(2) MODE OF LIFE. — Defence may be promoted by (a) Feeding
at favourable times, (6) Feeding in favourable places.

Supposing, however, that attacks are actually delivered, their
intention may be frustrated by means falling under a second large
heading.

II. RESISTANCE

This may take the form of either (A) Passive Defence, or
else (B) Active Defence.

(i) BODILY CHARACTERISTICS. — Of these perhaps the most
important are — (a) Unpalatableness and Indigestibility,
resulting from various peculiarities ; (b] Possession of

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 277

Armour, as purely defensive arrangements may be broadly
termed.

(2) SPECIAL HABITS, such, e.g., as the Death-feigning Instinct,
Rolling-up Instinct, &c.

(3) FECUNDITY as a means of defending the species.

(B) ACTIVE DEFENCE. — This employs the method of counter-
attack, and includes, among other things — (i) Use of ordinary
AGGRESSIVE WEAPONS; (2) Use of actively DEFENSIVE WEAPONS,
such as stings, stink-glands, &c. ; (3) CO-OPERATION for defensive
purposes between members of a community.

III. RETREAT

I. PRECAUTIONARY MEASURES

(i) BODILY CHARACTERISTICS.—

278 ANIMAL DEFENCES

arrangements now to be described are much less effective than
might at first sight be imagined. Aggressive animals would die
out altogether if their prey gradually evolved into a perfectly
protected condition, and such a state of things can never be
realized. Suppose a species A to feed upon other species B, c,
and D. Then as these develop characteristics which make them
more and more difficult to detect, A will keep pace more or less
with this by improvement of its sense organs and powers of
observation. Besides which, it does not follow that a protected
form which deceives the eye will necessarily deceive other senses,
such, e.g., as that of smell. This, however, is a matter which still
requires working out, for most attention has so far been paid to
arrangements which may metaphorically be called " optical
illusions ". And again, even if B, c, and D succeeded in evading
the pressing attentions of A altogether, that species might be
enabled to divert its energies in some other direction.

PROTECTIVE RESEMBLANCE (as arrangements tending to incon-
spicuousness may be called) may either be general, so as to
harmonize with the colour scheme, illumination, &c., of the sur-
roundings, or they may be special, so as to bring about an
imitation of some particular object, as a twig or leaf. It will
be convenient to consider these cases separately.

GENERAL PROTECTIVE RESEMBLANCE

TRANSPARENCY. — Vast numbers of marine animals belonging
to many different groups are to be found either swimming or
drifting in the surface layer of the sea, and it is characteristic of
such forms that they should be of glass-like transparency, which
obviously makes them very inconspicuous. Such an arrange-
ment, of course, only partly meets the situation, for the internal
organs of these "glass animals", to use the German expres-
sion, must of necessity be at least partially opaque, and many
of them may be condensed as it were into a limited region of
the body. But such organs, after all, look like bits of floating
weed. Good examples are furnished by free-swimming Ascidians,
such as Salps, many pelagic members of the Sea- Snail kind
(Gastropods), especially those which are known as Sea- Butterflies
(Pteropods) and Swimming- Snails (Heteropods), many Crustacea,
numerous Worms, and above all creatures such as Jelly -Fish
(Hydrozoa), together with the various species of Comb -Jellies

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 279

(Ctenophora). Transparency or translucency is a highly char-
acteristic feature in hosts of the free -swimming young which
belong to such groups as Molluscs, unsegmented and segmented
Worms, Lamp- Shells, Moss- Polyps, Crustacea, and Echinoderms.
A typical instance is that of the little Glass -Crabs which were
originally believed to be distinct species, but are now known to
be the larvae of Rock- Lobsters (Palinurus] and related animals.

SNOW ANIMALS. — Another striking and well-known instance
of general resemblance to surroundings is afforded by snow and
ice animals, which by possession of white fur or feathers are
rendered extremely inconspicuous. Mammals and Birds furnish
a number of striking examples. It is a rare occurrence for an
animal to be white all the year round, for this can only be ex-
pected to happen in very high latitudes, where the ground is
permanently covered by snow and ice. The Polar Bear is an
instance, but the colour in this case is almost entirely aggressive,
enabling the animal to approach its prey without attracting
observation.

Among birds the Snowy Owl (Nyctea Scandiacd] of the Arctic
regions, but which sometimes wanders into more southern lati-
tudes, as e.g., North Scotland, is permanently coloured so as to
harmonize with snowy surroundings. Its prevailing hue is white,
but this may be flecked or barred with blackish-brown. Though
in the main aggressive in purpose, the Owl is doubtless protected
to some extent by its resemblance to the background against
which it is usually seen in the far north.

DESERT ANIMALS. — Many animals which inhabit desert regions
exhibit general protective resemblance to their surroundings,
exemplifying in a practical manner the value of khaki tints
where concealment is desirable. The colour is by no means
always of uniform character, for there may be spots, flecks, or
stripes, all being calculated to make assimilation with the sur-
roundings more complete. Camels, Antelopes, Desert Foxes
(fennecs), and Jerboas are good instances among the Mammals,
while ancestral Horses were probably clad in striped khaki.
Among birds may be mentioned Desert Larks, Desert Finches,
and Sand-Grouse (fig. 474). Of the last, Brehm (in From North
Pole to Equator] writes graphically as follows, in reference to the
Sahara: — "Among the sparsely sprouting alfa there is a numerous
flock of birds about the size of pigeons. Tripping hither and

280

ANIMAL DEFENCES

Fig. 474. — Pallas's Sand-Grouse (Syrrhaptes paradoxus], native to the steppes of Asia

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 281

thither, scratching and scraping with their bills, they seek for
food. Without anxiety they allow the rider to approach within
a distance of a hundred paces. A good field-glass enables one
to see not only every movement, but also the more prominent
colours of their plumage. With depressed head, retracted neck,
and body held almost horizontally, they run about in search of
seeds, the few grains which the desert grasses bear, freshly-
unfolded panicles, and insects. Some stretch out their necks
from time to time and peer circumspectly around; others, quite
careless, paddle in the sand, preening their feathers, or lie at
ease, half-sideways, in the sun. All this one can distinctly see,
and one can count that there are over fifty, perhaps nearly a
hundred. What sportsman would their presence not excite?
Sure of his booty, the inexperienced traveller shuts up his field-
glass, gets hold of his gun, and slowly approaches the gay
company. But the birds disappear before his eyes. None has
run or flown, yet none is to be seen. It seems as if the earth
had swallowed them. The fact is, that, trusting to the likeness
between their plumage and the ground, they have simply squatted.
In a moment they have become stones and little heaps of sand.
Ignorant of this, the sportsman rides in upon them, and is startled
when they rise with simultaneous suddenness, and, loudly calling
and scolding, take wing and fly noisily away. But if he should
succeed in bringing one down, he will not fail to be struck by their
colouring and marking, which is as remarkable as their behaviour.
The sand -coloured upper surface, shading sometimes into gray,
sometimes towards bright yellow, is broken and adorned by broad
bands, narrower bars, delicate lines; by dots, spots, points, streaks,
and blurs, so that one might fancy at first sight that birds so marked
must be conspicuous from a distance. But all this colour-medley
is simply the most precise colour of the ground; every dark and
light spot, every little stone, every grain of sand seems to have
its counterpart on the plumage. It is no wonder then that the
earth can, as it were, make the bird part of itself, and secure its
safety, which is further assured by the creature's strong wings,
which are capable of incomparably swift flight."

A great many Lizards (see fig. 60) inhabit dry sandy places,
and present dull coloration in accordance with their surroundings,
as is the case with the common British Sand- Lizard (Lacerta
agilis], while numerous species are to be found in desert regions.

282 ANIMAL DEFENCES

Examples of these are the Thorn-tailed Lizards (Uromastix] of
North Africa and South-west Asia, which are burrowing forms
living entirely on plant-food, so that the coloration may be
regarded as entirely protective, while in species which prey upon
insects and other small animals it is aggressive as well. This
is the case, for instance, with the Desert Monitor ( Varamis
griseus), which has pretty much the same range as the last-
named species. It is a large animal, attaining a length of over
4 feet, and presents dark bars and streaks upon a neutral ground-
colour. Quite similar as regards general hue and character of
marking is the Common Skink (Scincus officinalis] of the Sahara,,
which, however, is only about 3 inches in length.

Our only poisonous British Snake, the Adder (Pelias berus}
(see vol. i, p. 233), commonly found on sandy heaths, exhibits
general resemblance to surroundings, which is both protective
and aggressive, as in the case of the Sand- Lizard. The ground
colour is usually brownish or greenish gray, and the back is
ornamented with a dark zigzag stripe. There are many groups
of true desert Snakes which are as inconspicuous as the Mammals
and Birds which inhabit the same regions, and here, too, as in
the Adder, the colouring serves a double purpose. A typical
example is the Horned Viper (Cerastes cornutus), receiving its
name from the presence of a pointed projection above either
eye in the male, and less often in the female as well. It inhabits
the deserts of North Africa and Arabia, and is of a sandy colour
with numerous dull blotches.

Desert Insects also exhibit the dull neutral tints and blotchings
or mottlings which prove so efficient in vertebrate forms. Ex-
amples are found in certain Desert Grasshoppers (Xiphocera asinay
Trachypetra bufo, and Methone Anderssoni] native to South Africa.

REVERSED SHADING. — General protective resemblance is not
merely a matter of appropriate colouring, but also of appro-
priate disposition of light and shade, and this is exemplified by
many of the cases already described, as well as by others to which
allusion will now be made. If such a curved white or brownish
solid as a common hen's egg be placed on a table and strongly
illuminated from above, it will be obvious that its upper part
will present high lights, while its under side will be in shadow,
the two regions gradually merging into one another. When a
drawing of an egg is made on a piece of paper, the intention is

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 283

to produce an optical illusion, whereby the effect of a solid on
the eye is simulated, and this is done by shading darkly the side
opposite to the one from which the light is supposed to come.
The exact opposite to this is brought about in nature as a pro-
tective device, and a solid object is made to appear more or
less flat by dark colouring above shading into light colora-
tion below, a very common state of things which has already
been briefly referred to (p. 271). A very ingenious model, of
which a copy exists in the Cambridge Museum of Zoology, has
been devised to demonstrate this. The back of a rectangular
box, of which the front and sides are glass, is marked with a
tangle of blotches and streaks to represent the confused mixture
of light and shade which may be seen in such a natural back-
ground as that presented by the sedges and other plants which
line the margin of a pond. A little in front of this a rod is
fixed from side to side, and to this are attached a couple of clay
models representing conventionally the bodies of two birds, with
the usual curved outlines. The upper parts of these are tinted
like the background and blotched in a similar manner, and are
gradually shaded off at the sides so as to pass insensibly into
the unshaded whitish tint of the under parts of the models. It
should further be stated that the rod can be rotated on its long
axis by means of a handle so as to move the diagrammatic
birds. Standing at a little distance from the model, before being
acquainted with its purpose, an average observer will almost cer-
tainly fail to see the dummy birds, but they at once flash, as it
were, into view when moved by means of the handle. The
device clearly illustrates why in so many animals the upper
surface is dark and the under surface light, and also shows that
a protected form must remain motionless if it is to derive much
benefit from its resemblance to surroundings. The principle is
illustrated in nature by such Mammals as Hares and Rabbits, by
almost any common sort of small Bird, by Lizards, Snakes, Frogs,
and innumerable Fishes. The last case is of particular interest,
for in such a form as, say, a Whiting, the distribution of colour
results in inconspicuousness as seen in side view, while the
darker upper side of the body is liable to be confounded (at
least in shallow water) with the background presented by the
sea-floor, and the pale under side is not particularly obvious when
looked at from below. It is clear that floating or swimming

284 ANIMAL DEFENCES

organisms thus derive an extra benefit from the lighter colour
of the under side, as, of course, they are liable to be attacked
from below, which is not usually the case with terrestrial forms.
Besides fish, other interesting instances of this device may be
taken from the larger pelagic forms which drift or swim at or
near the surface. These, Wallace states (in Darwinism) ". . .
are beautifully tinged with blue above, thus harmonizing with
the colour of the sea as seen by hovering birds; while they are
white below, and are thus invisible against the wave-foam and
clouds as seen by enemies beneath the surface. Such are the
tints of the beautiful nudibranchiate mollusc Glaiicus atlanticus,
and many others."

FLAT-FISHES. — It has just been shown that compact, curved
forms are often made to appear flattish by a particular distribu-
tion of colour and shading, but it is clear that similar advantages
might be secured in an entirely different manner, i.e. by the body
actually becoming flat. A striking case of this is that afforded
by the Flat- Fishes, such as Sole, Turbot, Plaice, and the rest.
These, when in the condition of young fry, swim about like
ordinary fishes, and possess the same bilateral symmetry. Very
soon, however, they become laterally flattened, and take to living
on the sea-floor, with either the left or right side downwards,
according to the species. This side remains white or pale, though
not for the reason given in the case of animals with pale ventral
surfaces, and its eye migrates to the side which is kept upwards ;
otherwise it would be of no use. And, further, the upwardly-
directed surface becomes darkly pigmented so as to harmonize
with the sand or mud upon which the animal lives, the resem-
blance often being enhanced, much as in desert animals, by the
presence of spots and blotches of darker or different tint. Here,
however, as in so many other cases, protection is only afforded
by the coloration and marking when the animal remains at rest.
As in the case of the model already described (p. 283), movement
at once destroys the illusion, and in the case of a flat-fish the
attention of other animals must be attracted by the display, to
a greater or less extent, of the white or light-coloured side. The
fish would be much better protected if both sides were dark, but
the development of pigment in the skin is bound up with the
action of light, which is largely excluded from that side of the
body which faces habitually downwards.

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 285

SPECIALIZED GENERAL RESEMBLANCE. — The animals so far
mentioned harmonize generally with the background against which
they are seen, but other instances are known where the form and
coloration are such as to make the creature appear part and
parcel of some special object upon which it lives. Such are
some cases which have been described of association between
certain Corals and Sea-Snails. There is, for example, a North
American Coral (Leptogorgia virgulata] which in shallow water
is of an orange-yellow colour, but of deep-reddish tint in deeper
water, the position of the individuals making up the colony being
marked by white spots. A Sea-Snail (Ovulum uniplicatuin) lives
upon the branches of this Coral, with which it harmonizes precisely
in colour, being orange-coloured in one case, red with white spots
in the other. Curiously enough, a similar close relation has been
noticed between a Coral and a Sea- Snail (Gorgonia verrucosa and
Ovulum patulum) which live in British seas, both animals being
red in colour. The Snail in this latter case (and no doubt in the
other as well) is doubly benefited, for the Coral is distasteful to
fish, and the Snail consequently has a good chance of escaping
the dangers of a too close inspection. Large specimens are often
to be seen in museums of a Coral (Gorgonia) which protects
Brittle-Stars in much the same way, these holding on to the
Coral by twining the tips of their slender arms round its branches.
A similar association is recorded between a rose-coloured Aus-
tralian Sponge and a small species of Sea-Slug. Sponges,' it
may be noted, are notoriously inedible, which is, of course, an
additional benefit to the mollusc.

GENERAL RESEMBLANCE IN EGGS AND YOUNG. — The harmon-
izing by means of form and colour with the usual surroundings, to
which the name of General Protective Resemblance has been given,
is by no means confined to adult forms, for it is exemplified by
various earlier stages in existence. These have perhaps been
more fully worked out in Birds and Insects than in other groups,
and one or two examples taken from these must suffice. Birds*
eggs, when laid in concealed places, free from observation, are
commonly white, but when exposed to view are more or less
coloured or marked, or it may be both. And the plumage of such
fledglings as run about on the ground is usually speckled or
mottled to harmonize with the surroundings, which renders them
extremely difficult to see, especially if the little creatures crouch

286

ANIMAL DEFENCES

Fig. 475. — Eggs of Ringed Plover {A£gUtttti> hiaticula]

close to the ground and remain motionless on the first note of
danger, as is commonly the case. Admirable instances of the kind
are afforded by the Ringed Plover (sEgialitis hiaticula) and

Kentish Plover (sE. Cantiana),
both of which lay grey eggs
with dark markings among
shingle on the sea-shore. In
these and their allies eggs and
young alike are exceedingly
difficult to detect (figs. 475,
476). Inspection of an admir-
able series of eggs and nests
with natural surroundings dis-
played in the British Museum
(Natural History) at South
Kensington will reward anyone
inclined to take an interest in
the matter. An apparent ex-
ception to the rule is afforded
by the eggs of Wood- Pigeons,
which, although exposed to
observation, are white in hue.
But it has been pointed out
that these are liable to be seen
from below through the chinks
in the nest, which may be
compared to a piece of open
basket-work, and are therefore
seen against the bright back-
ground of the sky. We are
reminded here of the light
under -surfaces of marine ani-
mals, which harmonize with a
similar background (p. 283).
Turning from Birds to Insects,, we find innumerable instances
of all the different stages in the life-history being rendered in-
conspicuous by coloration and markings which blend with the
immediate surroundings. Speaking of a native moth, Poulton
(in his delightful book on The Colours of Animals, from which
a number of the illustrations in this part of the subject are taken)

Fig. 476. — Crouching young of Peewit ( Vanellus cristatus]

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 287

remarks: — " The caterpillar, chrysalis, and moth of the Black
Arches (Psilura monacha] are beautifully protected in this way.
The black pupa is fixed in a chink in the bark by a few incon-
spicuous threads; its dark colour harmonizes with the shadow in
the chink, while the long tufts of greyish hair project and exactly
resemble the appearance of lichen. Both larva and moth are
coloured so as to resemble common appearances presented by

Fig. 477. — A Beetle (Lithinus nigrocristatus} which resembles Lichen

lichens, and both habitually rest on lichen-covered bark." Some
beetles exemplify the same device (fig. 477).

MASKING. — General protective resemblance to surroundings is
effected in a number of animals belonging to widely -different
groups by the presence on the surface of their bodies of plant-
growths or various foreign substances. Some small West Indian
Land- Snails, for instance, escape observation by reason of the dirt
with which their shells are covered, but among molluscs a much
more remarkable case is that of certain Sea -Snails (species of
Xenophorus) (fig. 478). Speaking of one of these, Chun (in Aus
den Tie/en des Weltmeeres] describes it as "a snail which pos-
sesses the remarkable habit of cementing to its shell in a sym-
metrical manner the empty shells of other snails. It might almost
be imagined that an artistic hand took part in grouping these
foreign shells." Bits of coral and stone may also be included.

Even more remarkable devices are adopted by some of the
Crustacea. There are, for example, various kinds of Spider-Crab
(species of Maia, luachus, Stenorhynchus, &c.) in which the body
is covered with a thick growth of sea- weed, and it has been shown
by Bateson and others that this is not an accident, for these

288

ANIMAL DEFENCES

creatures voluntarily convert the surfaces of their bodies into a
kind of sea- weed garden. " The Crab takes a piece of weed in
his two chelae, and, neither snatching nor biting it, deliberately
tears it across, as a man tears paper with his hands. He then

Fig. 478. — Upper side of a Xenophorus Shell

puts one end of it into his mouth, and after chewing it up, pre-
sumably to soften it, takes it out in the chelae and rubs it firmly
on his head or legs until it is caught by the peculiar curved hairs
which cover them. If the piece of weed is not caught by the
hairs, the Crab puts it back in his mouth and chews it up again.
The whole proceeding is most human and purposeful. Many
substances, as hydroids, sponges, Polyzoa, and weeds of many
kinds and colours, are thus used, but these various substances

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 289

are nearly always placed symmetrically on corresponding parts
of the body, and particularly long, plume-like pieces are fixed
on the head, sticking up from it " (Bateson).

Some Sea- Urchins render themselves very inconspicuous by
heaping bits of stone and shell upon their bodies. One would
be inclined to think their firm spiny shell quite enough by way
of protection, but it does not keep off star-fishes.

The last example of masking to be noted here is that of a
large red Sea -Anemone (Tealia crassicornis] which abounds on
our coast. The body of this creature is covered by sticky knobs,
to which small pieces of stone adhere. When uncovered by the
tide the animal draws in its tentacles and shrinks into a rounded
lump, which, in virtue of its extraneous covering, looks like a
little heap of gravel, and is commonly overlooked by those un-
aware of the facts of the case.

VARIABLE GENERAL RESEMBLANCE

The examples of protective resemblance so far quoted are
mostly permanent adaptations to one particular sort of surround-
ing. There are, however, numerous animals which possess the
power of adjusting their colour more or less rapidly so as to
harmonize with a changing environment.

COLOUR-CHANGE IN SNOW- ANIMALS. — Some of the best-known
of these cases are found among those Mammals and Birds which
inhabit countries more or less covered with snow during a part
of the year. A good instance is afforded by the Irish or Variable
Hare {Lepus variabilis\ which in these islands is chiefly found in
Ireland and Scotland. In summer this looks very much like an
ordinary Hare, though rather greyer in tint and smaller in size,
but in winter it becomes white, with the exception of the black
tips to the ears. Investigations which have been made on the
closely-allied American Hare (Lepus Americanus) seem to show
that the phenomenon is partly due to the growth of new hairs of
white hue, and partly to a change in colour which affects the tips
of the ordinary hairs. In both cases the whiteness would appear
to be the result of the presence of minute bubbles of carbonic acid
gas in the substance of the hairs themselves, they absorbing the
gas from their roots, where it would appear to be generated. The
Common Stoat (Putorius ermineus] (see figs. 315 and 316) is sub-
ject to similar colour -change in the northern parts of its range.

VOL. II. 51

29o ANIMAL DEFENCES

In summer it is of a bright reddish -brown colour, with the ex-
ception of the under parts, which are yellowish -white, and the
end of the tail, which is black. But in winter the entire coat,
save only the tip of the tail, becomes white, and in that condition
the animal is known as an Ermine. A similar example is afforded
by the Weasel (Mustela. vulgaris\ The seasonal change in the
vegetarian Irish Hare is purely of protective character, but in such
an actively carnivorous creature as a Stoat or Weasel it is aggres-
sive as well, rendering the animal inconspicuous to its prey.

Among Birds no better example can be selected than the
Ptarmigan (Lagopus mutus) (fig. 479), native to the Scottish

Fig. 479. — Ptarmigan (Lagopus mutus), in winter plumage

Highlands among other places. The summer plumage of this
bird is brown with darker markings, but in winter it is pure white,
except that the outer tail-feathers remain black, and the male bird
retains a black band in front of the eye.

COLOUR-CHANGE IN CHAMELEONS. — The power of adjusting its
colour to suit the surroundings in the Chameleon is almost pro-
verbial, and the change here takes place with extreme rapidity.
As in the Stoat, it is quite as much aggressive as protective in
nature. The colour -change in this case is brought about by
alterations in the size of pigment -holding cells contained in the
deeper part of the skin. Under the influence of the nervous
system these can either be contracted to mere spots, or relaxed
into branching star-like forms. Since they are arranged in a
number of layers and contain different sorts of pigment, it is

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 291

clear that the colour effect will vary according to the number
which are at a given time in a relaxed condition, and the depth
in the skin at which these are situated. And it has further
been proved that the eyes of the animal must be uninjured if the
hues of the body are to be adjusted, so that the colours of the
surroundings first affect the visual organs, from which nervous
impulses are carried to the brain and thence in a roundabout
way to the skin. It will readily be perceived that a case like
this presents a physiological problem of no little complexity, the
nature of which has only been indicated. It may be here re-
marked that though the Chameleon is the most striking example
among the Reptiles of colour-change, it is by no means the sole
member of that class which illustrates the phenomenon.

COLOUR-CHANGE IN AMPHIBIANS. — Some familiar Amphibians
vary in colour according to the surroundings, though not so
rapidly as the last-named animal, and the nature and causes of
the change are substantially the same. Our Common Frog (Rana
temporaria] has been the subject of careful investigations in this
respect. It is quite easy to produce the change in a captive
specimen. If placed for a short time in a dark box it will assume
a blackish appearance, and if then transferred to fresh damp grass
and placed in a bright place it will speedily take on a yellowish-
green tint. The arrangement is obviously protective, but it may
be aggressive as well, for insects form a large part of the food,
and a good dinner may depend upon approaching them unobserved
until the long tongue can be shot out with a good chance of catch-
ing them (see p. 82).

COLOUR-CHANGE IN FISHES. — Fishes, especially those which
are ground-feeders, possess in many if not all cases a power of
colour-adjustment, the mechanism being the same as in Reptiles
and Amphibia. The protective arrangements in Flat- Fishes have
already been noted (p. 284), but probably the description might
have been given with greater propriety under the present heading.
Plaice, for example, are known to be able to vary their prevailing
hue according as the surrounding part of the sea-bottom is light
or dark, and a very interesting observation was made many years
ago upon a number of these fishes which at the time were resting
upon white sand. With the exception of one specimen they had
all assumed a light colour, and on closer examination that par-
ticular fish proved to be blind, and therefore did not possess the

292 ANIMAL DEFENCES

power of colour - adjustment. Similar observations have been
made in the case of Trout. Many other fishes are well known
to vary in hue according to their surroundings, and the present
writer remembers seeing a striking case some years ago in a
marine form known as the Lumpsucker (Cyclop terus lumpus).
This fish is a ground-feeder, and an example had been caught at
the floating marine station then existing at Granton, near Edin-
burgh. It was of a forbidding black appearance, and to keep it
alive for further observation was suspended in the clear sea- water
within a sort of drum made of net- work, the top of this being just
above the surface. After a short time the fish was pulled up for
re-examination, and was then found to be of a bright-green colour,
strikingly different from its former sooty hue, and harmonizing
well with the sea-water.

It has been proved for Frog and Chameleon, and is no doubt
true for fishes as well, that the colour-changes are brought about
by various external agents which affect the nervous system, these
including not only light but also changes in temperature, contact
with surrounding objects, and variations in the amount of oxygen
available for breathing.

COLOUR-CHANGE IN MOLLUSCS. — Some of the Sea-Slugs
exemplify variable protective resemblance. This is the case,
for example, with Elysia viridis, a little creature closely re-
sembling a Land - Slug in form and method of progression.
Those specimens found among green sea-weeds are green in
colour, while brown weeds of various hue harbour individuals
which harmonize with their particular tint. (See as to what
has been said on p. 285 regarding Ovulum.)

COLOUR-CHANGE IN CRUSTACEANS. — Many of the smaller
Crustaceans which are translucent, or it may be transparent,
afford most instructive examples of variable protective resem-
blance. The Common Prawn (Palczmon serratus), for example,
has long been known to become dark in colour when placed on
a dark surface, and colourless when allowed to rest on a white
surface, but by far the most remarkable case so far investigated
is that of the ^Esop Prawn (Hippolyte variant], a species which
is common in low-tide pools and shallow water. This has been
the subject of an elaborate research by Keeble and Gamble, who
have confirmed and extended the results of several previous
workers. It is found that these Prawns harmonize in the most

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 293

perfect manner with the weeds among which they happen to be
found, green, red, or brown as the case may be; and further, that
individuals adapted to live among weed of one colour are able
to adjust their appearance, though not very rapidly, to surround-
ings of new form and colour. There is further a quick response
to alterations in the intensity of light. But beyond this there is
a remarkable periodic colour-change, corresponding to the alter-
nations of day and night, the utility of which is not at present
known. To quote the above-named authors: — " Every evening,
as darkness comes on, Hippolyte gradually loses its distinctive
diurnal colour. In summer the change begins at about 9 P.M.,
in winter at about 5 P.M. Towards this or that time, according
to the season, a reddish tint — a sunset glow — the foreshadowing
of the change, makes its appearance. This is followed by a green
tinge which spreads fore and aft from the middle of the body.
The green colour gradually melts into blue, and a general increase
of transparency sets in. Thus, as darkness falls, Hippolyte is
seen to become of a wonderful azure -blue colour and absolutely
transparent, except in the region of the liver and stomach, which
are now very clearly visible. The depth of the blue colour varies
in different specimens; in some it is almost indigo, in others the
faint azure of a sky at sunset." The changes in hue depend upon
the condition of branching colour-bodies which are present. These
contain various pigments, the distribution of which can be varied
as the result of the action of light and other agents, that work
in part through the agency of the eyes and nervous system, and
in part independently of these.

COLOUR-CHANGE IN INSECTS. — The subject of variable pro-
tective resemblance must be concluded by referring to some
extremely interesting cases presented by Insects in the various
stages of their life-history. Numerous instances are known where
caterpillars of the same kind vary in colour according to the hues
of their surroundings. This may be illustrated by an experiment
of Poulton's conducted on the larvae of the Peppered Moth
(Amphidasis betularia). A large batch of eggs taken from the
same individual was divided into two parts, one half being then
hatched out among the green leaves and shoots of birch, and
the other among dark-brown twigs of the same plant with a
certain admixture of leaves. All the caterpillars of the former
half were bright -green, while the large majority of the other

294 ANIMAL DEFENCES

half were dark-brown, though about one or two per cent took their
colour from the leaves present. Adaptability to surroundings as
regards appearance is exemplified not only by caterpillars but also
by the chrysalides of some moths and butterflies. No better
example could be selected than the Small Tortoise-shell Butterfly
(Vanessa urticcz], upon which Poulton conducted a remarkable
series of experiments. By varying the surroundings at the time
when the caterpillars become quiescent preparatory to passing
into the motionless pupa-stage, he was able to produce at pleasure
dark, light, and gold-coloured chrysalides. The utility of a gilded
appearance is not at first sight obvious, but it may be pointed
out that it would harmonize with a rock-surface which presented
such glittering minerals as mica (flakes of which are used on
Christmas trees and the like to give the glistening appearance of
snow). Rock-surfaces in our damp climate are generally dull,
except when freshly fractured, but in hotter and drier countries
they often present a glittering appearance, and in this connection
it is interesting to note that the word chrysalis, which means
golden (Gk. chrysos, golden), is taken from Aristotle, and was
no doubt given from the appearance of certain pupae noticed by
the ancient Greeks. Possibly, therefore, the power of producing
gilded chrysalides possessed by the caterpillars under discussion
may be reminiscent of a time when the ancestors of the Small
Tortoise-shell Butterfly inhabited more southern latitudes than
ours. The range of this particular species at the present time
includes a good deal of the northern hemisphere, and the distri-
bution of the family to which it belongs (Nymphalidae) is world-
wide, so that there is nothing improbable about the suggestion.

SPECIAL PROTECTIVE RESEMBLANCE

Here are included cases where inconspicuousness results
from resemblance to some special inedible object, instead of
being due to properties of form and colour whereby harmony
with the general surroundings is brought about. There is, how-
ever, no sharp boundary-line between these cases; e.g. a brown
-^Esop Prawn may not only assimilate generally to the appearance
of the weed to which it clings, but may also simulate a special
part of this. Special Protective Resemblance, like the other form
of assimilation, may be either constant or variable.

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 295

CONSTANT SPECIAL PROTECTIVE RESEMBLANCE. — A good in-
stance on the border line between general and special resemblance
is afforded by some of the Sloths, which hang head downwards
from the branches of trees in South American forests. The harsh
greenish-looking fur of these creatures harmonizes very completely
with the moss and lichen by which they are commonly surrounded,
and it is very interesting to note that the hairs are grooved or
fluted in a peculiar way so as to afford a lodgment to a microscopic
green plant (alga) which, favoured by the humidity of the air, is
able to grow upon them. But, in addition to this arrangement
promoting general resemblance, there is another bringing about
special resemblance, in the form of a round fawn-coloured patch
between the shoulders, and this is the more conspicuous because
it has a dark margin. The effect is similar to that produced by
a rotten branch which has broken off short, leaving a stump with
a light centre constituted by the wood, and a dark margin due to
the fractured bark.

Another instance among Mammals which may more properly
be referred to special resemblance is afforded by the Pangolin
(Manis], a scaly arboreal Edentate native to East Africa and
Southern Asia. If alarmed when climbing a tree -trunk, this
animal lets go with its fore-limbs, and, supporting itself by hind-
limbs and tail, presents an appearance quite comparable to a
broken branch.

Among Birds, the Coot, Moor- Hen, and Grebes may possibly
be taken, though the matter is not beyond cavil, as illustrations
of some particular part being shaped so as to confer special
protective resemblance. The toes of these birds are broadened
out in a very curious manner, and it has been suggested that
this gives them a certain resemblance to the leaves of floating
water-plants, whereby rapacious fishes are deceived and refrain
from nibbling at them. Too much stress must not be laid on
this, however, as it may be merely a case of broadening for the
purpose of increased efficiency in swimming. Or possibly two
ends may be gained at the same time.

There are also cases of young birds, belonging to species in
which the eggs are laid and hatched out in exposed situations,
where the colour and markings not only harmonize generally
with the surroundings, but also bring about a protective resem-
blance to a single stone when the crouching attitude is assumed

296 ANIMAL DEFENCES

on a sudden alarm. This is the case, for instance, with the
young of some Plovers (see p. 286).

A good instance of special protective resemblance among the
lower Vertebrates is afforded by Australian species (Phyllopteryx
eques, and two others) of the curious fishes known as Sea-
Horses, of which a less remarkable genus (Hippocampus} is
commonly represented in museums. These Australian forms,
which may be as much as a foot long, are laterally flattened,
and attach themselves by means of their curly tails to pieces of

Fig. 480. — Australian Sea-Horse (Phyllopteryx eques]

brown sea-weed (species of Fucus). Glinther (in The Study of
Fishes) says of them: — " Not only their colour closely assimilates
to that of the particular kind of sea- weed which they frequent, but
the appendages of their spines seem to be merely part of the fucus
to which they are attached". (Fig. 480.)

Herdman and others have described a number of very inter-
esting cases of protective form and colour among Sea-Slugs.
Some of these (notably Doto coronata and Dendronotus arbor-
escens) have their upper surfaces richly studded with brightly-
coloured projections (cerata), which at first sight serve to make
them conspicuous, but in reality so closely resemble the branch-
ing colonies of zoophytes among which they live as to make them
difficult for their enemies to detect.

Many of the most striking of the special protective resem-
blances so far described are to be found among Insects in various

BODILY CHARACTERISTICS— INCONSPICUOUSNESS

297

stages of their existence. Among our native Moths, for instance,
there is one group (Geometers] containing over 200 species, in
which the succulent caterpillars move along in a curious way
which has earned for them the name of Loopers, and is also
the origin of the name of the group. The front part of the
body bears three pairs of jointed legs corresponding to those
of the future moth, while at the hinder-end are two pairs of
sucker-like pro-legs. It is by alternate use of these fore and aft
groups of limbs that the characteristic movements are effected.
But the larvae in question have also earned the name of Stick-
Caterpillars, from the very perfect way in which they simulate
resemblance to the twigs of plants on which they live. Holding
on firmly by means of its pro -legs, and extending the body
obliquely outwards, such a caterpillar remains motionless and
rigid for hours, until the cravings of hunger render it necessary
to crawl away and take the next meal. Such a severe trial of
patience and muscular power are rendered rather less trying
by a silken thread attaching the head to a neighbouring twig.
A Stick-Caterpillar in the motionless condition harmonizes so
perfectly with its surroundings, both as regards shape and colour,

that it can only be detected by an

unusually keen or practised observer.

Poulton (in The

Colours of Ani-

mals) describes

as follows one of

the most remark-

able cases: — " I

will illustrate the

extraordinary de-

gree of resem-

blance attained

in Geometrce by

a description of

fu 1 of one Fig. 482. -Hinder End of Caterpillar of

ttl( laFVa C Brimstone Moth (magmfied)

of our most abun-

dant species, the Brimstone Moth (Rumia crat&gata). The appear-
ance of the larva when seated among the twigs of its commonest
food-plant — hawthorn — is shown in fig. 481. It will be observed
that some of the twigs are slightly bent in the middle, and that

Fig. 48i.-Caterpillars of Brimstone Moth
(Lmia crataiata], in protective attitudes

298

ANIMAL DEFENCES

a projection is placed on the angle; these appearances are
exactly reproduced in the larva. The hind part of the larva
is represented in fig. 482 (magnified 4*5 diameters), showing
the claspers and the fleshy projections which occupy the furrow
between the larva and the stem. The harmony of colour is
quite as perfect as the resemblance of shape. The smaller
branches of the hawthorn are partially covered by a thin super-
ficial layer of a bluish-grey colour (the cuticle), while the deeper
layers beneath are brown or green, or mixed brown and green;
these tints become visible over a large part of the surface, owing
to the breaking away of the thin layer. Hence the colour of
the branches is brown or green, mottled with grey, and not only
are these the exact tints of the larva, but the way in which the
colours are blended is precisely similar in the animal and the
plant. The marvellous fidelity with which the details are thus
reproduced probably implies that the relation between the larva
and this species of food-plant is extremely ancient. . . . This
caterpillar can also adjust its colour to that of its individual
surroundings, so that it would become greenish if it passed its
life among young green shoots, and brown if it lived upon the
older twigs. It is altogether one of the most perfectly-concealed
forms in existence."

No less remarkable cases of protective resemblance are found
among adult insects, one of the most perfect (fig. 483) being that

Fig. 483. — Indian Leaf Butterfly (Kallima inachis] with wings extended, and on twig in protective attitude

presented by certain Oriental and African Leaf- Butterflies (species
of Kallima) described by Wallace. When on the wing these
insects are conspicuous objects, owing to the bright orange and

BODILY CHARACTERISTICS— INCONSPICUOUSNESS 299

purple tints which adorn the upper sides of their wings. Ob-
servers who have watched them in the dry forests which they
haunt, call attention to their rapid flight, and the sudden way
in which they settle on twigs and in the twinkling of an eye
transform themselves into the semblance of withered leaves. In
this resting attitude the wings are folded together so as to con-
ceal the bright colours and expose their under sides, which in
both colour and markings harmonize in the minutest details with
the objects they resemble, even to the presence of spots com-
parable to diseased patches. On each fore-wing two small round
areas are devoid of scales, leaving the transparent membrane
quite bare, and when these two spots are applied to their fellows
an appearance is brought about comparable to a hole, such as
might be produced in a leaf by the attack of an insect larva.
Antennae, head, and body are hidden between the folded wings,
which offer a continuous outline just like that of one of the neigh-
bouring dead leaves, the fore- wings being pointed in front, and
the hind-wings backwardly produced into narrow tails which pass
muster as a leaf-stalk. The legs of the Butterfly are so slender as
not to destroy the illusion.

Many of our native moths are so coloured and marked as to
be readily mistaken for dead leaves, and one kind, the Buff- tip
Moth (Pygara bucephala), is, when at rest, a very perfect counter-
part of a bit of rotten stick which has snapped across so as to give
a flattish yellowish-brown surface at the end.

The Stick- and Leaf- Insects of the Locust and Grasshopper
order (Orthoptera) closely resemble the objects after which they
are named. Sharp (in The Cambridge Natural History) remarks
of the latter: — " The resemblance presented by different kinds
of Orthoptera to leaves is so remarkable that it has attracted
attention even in countries where Natural History is almost totally
neglected; in many such places the inhabitants are firmly con-
vinced that the Insects are truly transformed leaves, by which
they understand a bud developing into a leaf and subsequently
becoming a walking-leaf or Insect. To them the change is a
kind of metamorphosis of habit ; it grewr as a leaf and then took
to walking."

Spiders also, in a number of cases, closely resemble special
objects, the end being in this case aggression as well as pro-
tection. There is, for instance, a Mascarene species (Carosfais

3oo ANIMAL DEFENCES

mitralis) which when at rest might readily be mistaken for a
projecting knot, while other sorts have been described in which
splashes of birds' excrement are faithfully copied.

The foregoing examples will perhaps suffice to illustrate pro-
tective resemblance more or less constant in kind to special
objects, and a few instances will now be given of the somewhat
rarer phenomenon of variable resemblance of the same kind.

VARIABLE SPECIAL PROTECTIVE RESEMBLANCE. — So far as the
species is concerned, the Leaf- Butterflies (Kallima) already
described might very well be placed here, for there is great
variability in the colouring of individuals, just as there is among
the dead leaves to which the colours give resemblance. But cases
are also known where the same individual is capable of altering its
appearance so as to copy, as it were, more than one sort of ex-
ternal object. The power of doing this is suspected in far more
numerous instances than those for which it has actually been
proved, but Poulton (in The Colours of Animals] gives a strik-
ing and unequivocal example in the stick -caterpillar, which is
the larval stage of the Early Thorn Moth (Selenia illunaria),
a native species. When resting on a brown twig this cater-
pillar conceals itself in the way already described for a similar
form, but when feeding upon a green leaf that method would
be impossible. Yet by throwing its body into several sharp
kinks, and remaining motionless, it assumes a resemblance to a
shrivelled bit of dead leaf or some other object such as might be
expected to occur on the foliage.

We have now discussed at some length cases of precautionary
measures (see p. 276) depending upon bodily characteristics which
make for inconspicuousness, and we must now turn to instances of
precisely opposite kind, where bodily characteristics make their
possessor very conspicuous.

CHAPTER XXVI

ANIMAL DEFENCES— BODILY CHARACTERISTICS
PRODUCING CONSPICUOUSNESS

CONSPICUOUSNESS AS A MEANS OF WARDING OFF ATTACKS.

Numerous animals are possessed of stings, poison-glands, stink-
glands, and the like, which are capable of making them disagree-
able or dangerous to their enemies, and many such creatures are
practically inedible. But as at close quarters they might be killed,
even if not eaten, a device for advertising noxious qualities has
been evolved in the form of what is known as WARNING COLORA-
TION, to which are closely allied certain other warning methods
independent of colour-effects. No doubt a considerable number of
these conspicuous forms must fall victims to the attacks of inex-
perienced aggressors, who, however, taught by experience, are not
likely to continue their investigations, so that the warning species
is on the whole benefited. And the welfare of the individual is
always subordinate to that of the species.

t Warning devices carry in their train a very extraordinary
phenomenon, for many perfectly harmless and edible kinds of
animal trade upon the evil reputation, so to speak, of warning
noxious forms, by coming to resemble these in a very detailed
manner. This is technically known as PROTECTIVE MIMICRY,
though of course the imitation is a purely unconscious one.
Mimicking forms enjoy considerable immunity from attack, by
means of sailing under false colours. It will be most convenient
to consider the phenomena involved by warning under two
separate headings: GENUINE WARNING and SPURIOUS WARNING
(Mimicry).

GENUINE WARNING

The best -known cases of such warning are found among
lower forms, but one typical case within the class of Mammals is
known in the person of the American Skunk (Mephitis suffocant)
(fig. 484), a creature that possesses glands the secretion of which

301

302

ANIMAL DEFENCES

exhales a most disgusting and penetrating odour. In colour it
is black and white, so distributed as to make it extremely con-
spicuous, for the ^lpper side is white instead of the lower, as in

Fig. 484.— American Skunk (Mephitis suffocans]

cases where inconspicuousness is the end to be attained (see p.
282). The possession of a large bushy white tail, which is
carried erect, makes the animal still more easy to see, and it is
in ^the habit of progressing at an easy pace, which it does not
quicken even when attack is threatened. Should these danger-
signals be ignored by an inexperienced enemy, the Skunk defends

BODILY CHARACTERISTICS— CONSPICUOUSNESS 303

itself very effectively by ejection of the noxious fluid already
mentioned, and of which Hudson says (in The Naturalist in La
Plata)'. — " Men have been blinded for ever by a discharge of the
fiery liquid full in their faces. On a mucous membrane it burns
like sulphuric acid, say the unfortunates who have had the ex-
perience." The same author quotes from the Ibis a description
given by Mr. Ernest Gibson of an encounter between an eagle-
vulture (Polyborus tharus] and a skunk. " Riding home one after-
noon he spied a skunk ' shuffling along in the erratic manner usual
to that odoriferous quadruped ' ; following it at a very short dis-
tance was an eagle-vulture, evidently bent on mischief. Every
time the bird came near, the bushy tail rose menacingly; then the
caranco would fall behind, and, after a few moments' hesitation,
follow on again. At length, growing bolder, it sprung forward,
seizing the threatening tail with its claw, but immediately after
' began staggering about with dishevelled plumage, tearful eyes,
and a profoundly woebegone expression on its vulture face.' The
skunk, after turning and regarding its victim with an I-told-you-so
look for a few moments, trotted unconcernedly off."

A number of Poisonous Snakes exemplify warning by colour
or other means, and the reason for this is not immediately ob-
vious, for these creatures possess very efficient defensive weapons
in their fangs. But, as Poulton points out (in The Colours of
Animals], snake -poison does not kill immediately, and the
aggressor would have time to despatch his quarry before suc-
cumbing to it. Besides which, the amount of this poison available
for the time being is but small, and a snake which has used its
fangs is thereby left for some time in a comparatively helpless
condition.

The Coral-Snakes of Asia, Africa, America, and Australia are
among the best examples of Reptiles exhibiting warning colora-
tion. One of the most striking species (Elaps corallinus), native
to South America and the West Indies, is of a bright-red colour,
marked with broad black rings, the margins of which are greenish-
white.

Some snakes employ terrifying or warning attitudes as a
means of warding off attack, notably the Cobras, which, when
threatened, raise the front part of the body from the ground and
inflate the neck, on the back of which is a prominent spectacle-
like marking. The Puff-Adders of Africa swell up their bodies

3°4

ANIMAL DEFENCES

under similar circumstances, and this possibly prevents other
animals from assaulting them.

Many poisonous serpents are accredited with producing terri-
fying or warning sounds, which may or may not be associated
with the other warning methods above described. The character-
istic hissing is probably a
case in point, and very pro-
bably the rattle of the pro-
tectively - coloured Rattle-
snake may answer the same
purpose (fig. 485).

It is well known that the
skin of Amphibians is pro-
vided with numerous small

Fie. 485. — Rattle of Rattlesnake, a, In section; b, exterior - - r 1 . i i

glands of which the secre-
tion is more or less poisonous, and the common Toad, for example,,
is on this account treated with a fair amount of respect by Dogs,,
Cats, and the like.

There are certain species of Amphibia where these poisonous,
properties are advertised by the presence of warning colours. The
conspicuous black -and -orange hues of the Spotted Salamander
(Salamandra maculosa) may be of this nature, though a clearer
case is afforded by a small red-and-blue frog described by Belt
(in A Naturalist in Nicaragua). The observer mentioned, sus-
pecting from its bold demeanour and glaring colours that this
species was inedible, found by experiment that it was invariably
rejected by fowls and ducks. A still more interesting case has
recently been described by Annandale (in The Proceedings of the
Zoological Society of London, 1900) of an Amphibian which only
shows bright colours when alarmed. This is a species of Toad
(Callula pulchrd] "which is found not uncommonly in the Siamese
States, among the rubbish which collects under the houses and
in like situations. In this species, the upper surface of which is
otherwise of a warm brown colour, a broad yellowish stripe runs
along either side of the back; but the peculiar looseness of the
skin and the folds into which it naturally falls prevent this stripe
from becoming conspicuous. When the animal is disturbed, how-
ever, it draws air into its lungs until its body becomes almost
globular, and the skin is stretched in such a way that its con-
trasting colours are displayed to their best advantage." This and

BODILY CHARACTERISTICS— CONSPICUOUSNESS 305

the examples already given will show that warning colours are
of such a kind and arranged in such a way as to secure con-
spicuousness, a fact which will be exemplified by still further
instances. The primary colours, especially red and yellow, are
the commonest in this connection, and the patterns in which they
are arranged commonly include spots, bands, and blotches.

Hudson (in The Naturalist in La Plata) gives a vivid account
of the curious Horned Toad (Ceratophrys ornata) (fig. 486) of
South America, an aggressive creature in which the poisonous

Fig. 486. — Horned Toad (Ceratophrys ornata}

secretion is unusually virulent. Bright -green in colour, with
chocolate patches and yellow lips, this Toad is unusually for-
bidding in appearance, and very conspicuous, except when, half-
buried in some damp spot, it lies in wait for the small vertebrates
upon which it feeds. " When teased, the creature swells itself
out to such an extent one almost expects to see him burst; he
follows his tormentors about with slow awkward leaps, his vast
mouth wide open, and uttering an incessant harsh, croaking
sound."

It is probable that brilliant or striking coloration in some
Fishes is to be correlated with poisonous or other unpleasant
qualities, but the matter still requires working out. Garstang
(quoted by Poulton in The Colours of Animals] thinks that an
example is afforded by our native Weever-Fish (Trachinus
vipera], which possesses poison-spines on its gill-covers, and is
distinguished by the intense black colour of its first dorsal fin.
It is in the habit of burying itself in the sand, this fin alone

VOL. II. 52

306 ANIMAL DEFENCES

projecting, perhaps as a warning to Gurnards and other pre-
daceous Fishes, which prey upon the Dragonet (Callionymus
fyra), that is not unlike it and has similar habits, though quite
devoid of poisonous properties. Annandale (in the paper already
quoted) applies this explanation to the Globe -Fishes (genus
Tetrodon, &c.), "which have earned the name of Balloon- Fish
among Europeans, and . . . Pillow- Fish among Malays, by the
manner in which they gulp down air into their stomachs, so
causing the brilliant coloration of many of them to become con-
spicuous, and also the spines with which they are armed to be
erected."

Numerous instances of warning coloration are to be found
among the Protochordates. Some of the compound Ascidians,
for example, are very brightly coloured, and this is associated
with an unpleasant odour. This correlation of facts suggested
to Garstang that they are probably unpalatable to Fishes, and a
series of experiments conducted by him proved this actually to
be the case. A better example of the scientific method could
scarcely be found: hypothesis based on facts, and then confirmed
by experiment.

The different species of Acorn-headed Worms (Balanoglossus)
are more or less brightly coloured and endowed with an unplea-
sant odour, which makes it probable that they too are possessed
of unpalatable qualities.

Warning Colours of Molluscs. — The bright tints of certain
Gastropods undoubtedly have a warning significance, though, as
in other cases, it is necessary to watch the animals in their natural
surroundings before coming to a definite conclusion, for colours
and colour-schemes which in themselves are extremely striking
may notwithstanding be well adapted to effect concealment when
their possessor is "at home". A good example described by
Herdman is that of the Sea-Slug (Nudibranch) Eolis, the upper
surface of which, as in so many of its kind, is beset with tentacle-
like projections (cerata) of brilliant colour, which in this particular
case do not harmonize with the natural surroundings. When it
is added that the tips of the cerata are provided with stinging cells
much like those of Jelly- Fishes and Sea- Anemones, it is not
surprising to learn that actual experiment proved Eolis to be
unpalatable to fishes.

Striking colours were proved by Garstang to be associated

BODILY CHARACTERISTICS— CONSPICUOUSNESS 307

with inedibility in another Sea- Slug (Pleurobranchus membrana-
ceus), which is distinguished by its conspicuous yellow and reddish
markings, while its skin secretes an acid. The two examples
given will suffice for purposes of illustration, but the bright colours
of many other Gastropods, and perhaps also of some bivalve
molluscs, may very likely be explained on the same lines.

Warning Colours of Insects. — So many cases have been de-
scribed of genuine warnings among Insects of various kind, that
the difficulty here lies in the selection of material. Good examples
are to be found among those members of the Hymenoptera which
possess a sting, as, e.g., Wasps and Hornets, which are rendered
very conspicuous by alternating rings of black and yellow. Similar
markings and colours are exhibited by many Bees, though in their
case red is often substituted for yellow.

The largest Butterfly to be found in Britain is the Black-veined
Brown (Anosia erippus\ an American form which appears to be
doing well in the struggle for existence, for its area of distribution
is steadily getting wider. The wings are of an orange-brown
colour, marked with conspicuous black veins, and edged with
black upon which white spots are displayed. The under side
is as conspicuous as the upper, contrary to the rule for most
Butterflies. This form is known to be distasteful to insectivorous
animals, and what is even more interesting, Scudder has shown
that its eggs and caterpillars are not attacked by certain parasitic
insects that play an important part in keeping down the numbers
of many other Butterflies and Moths.

The Magpie Moth (Abraxas grossulariata] is a good example
of a British species possessed of distasteful qualities, and warn-
ingly coloured in all three stages of its existence. The caterpillar
presents black -and -orange markings on a cream ground, the
chrysalis is black with yellow bands, and the wings of the perfect
insect are yellowish-white with conspicuous black spots.

Numerous experiments have been conducted by several natural-
ists regarding the edibility or otherwise of a large number of
caterpillars, and as the result of these it may be stated as a general
rule that insectivorous animals such as Lizards, Birds, &c., reject
conspicuously-coloured forms, but eagerly devour those which are
more or less inconspicuous in their natural surroundings. It may
therefore fairly be concluded that the colours and markings which
make certain caterpillars easy to see are of a warning nature. As

3o8 ANIMAL DEFENCES

regards adult Butterflies and Moths, colours and markings of the
kind may either be genuine warnings or belong to the category
of spurious warnings, which will be described in the sequel.

Certain unpalatable Beetles exhibit warning colours, and of
these good examples are afforded by the various species of Lady-
bird (Coccinella), which exhibit red spots on a dark ground.

Warning Coloitrs of Arachnida. — The colours of this group
have not been studied in the same detail as those of Insects, and
perhaps the best-known case of warning is that afforded by a
black-and-red Australian Spider (Latrodectus scelid), which has
already been described (p. 127).

Warning Colours of Lower Invertebrates.— There can be no
doubt that the bright hues of many marine Bristle-Worms have
a wrarning significance, though we have, at present, but little
evidence on the subject. There is, however, at least one clear
case, i.e. Poly cirrus aurantiacus, a bright-red worm with very
numerous long tentacles of an orange colour, which has been
the subject of investigation by Garstang. This animal belongs
to a family (Terebellidte) of tube-inhabiting worms, but has given
up this mode of life, and is found crawling on stones and sea-weed
in pursuit of food. When alarmed it coils itself up so as to be
surrounded by its tentacles, and it was shown that these are very
distasteful even to voracious sorts of fish. It is an interesting fact
that when irritated these tentacles gleam with a phosphorescent
light, and this no doubt serves as a warning to the numerous fishes
which are in the habit of feeding by night. It would be unsafe,
however, to conclude from this and similar cases that the wide-
spread phenomenon of phosphorescence always has a warning
significance. It no doubt, just like colour, serves various ends
(see p. 86), though it cannot be said that its function is well
understood in a large number of cases.

Among unsegmented worms it is probable that some of the
brightly- coloured conspicuous Planarians, which do not attempt
to conceal themselves, possess noxious properties, and Gamble
(in The Cambridge Natural History] suggests that this may be
the case with two large Mediterranean forms, of which one
(Pseudoceros velutinus) is jet-black, and the other (Yungia
aurantiaca) bright-orange in hue.

Sea -Anemones and Coral -Animals are often of the most
brilliant colours, and these may well be of warning nature, for

BODILY CHARACTERISTICS— CONSPICUOUSN ESS 309

the stinging-cells with which these creatures are richly endowed
make them very undesirable morsels. The bright hues of many
Jelly-Fishes are similarly associated with cells of the sort, and
these creatures have been aptly styled "sea-nettles". Although
Sponges are not provided with stinging - cells they are often
extremely malodorous, and the sharp limy or flinty spicules which
abound in their tissues cannot add to their acceptability as an
article of diet. Their unpleasant properties are commonly asso-
ciated with bright colours, orange being a favourite tint.

SPURIOUS WARNING OR MIMICRY

Genuine Warning having now been briefly considered, we may
turn to a class of facts coming under the head of Spurious Warning
or Mimicryo This embraces cases where an animal devoid of any
noxious qualities resembles in form or colour, or both, some other
creature which is notoriously unpalatable and advertises this fact
by means of warning coloration. It is scarcely necessary to recall
a statement made elsewhere to the effect that the so-called mimicry
is a purely unconscious process. We are here only concerned
with the fact that such mimicking forms would appear to benefit
more or less by their resemblance to such well-defended species,
being probably treated with respect by natural enemies which would
otherwise be pressing in their attentions. It may, however, be as
well to state here that there has been too great a tendency to
assume offhand that given animals are illustrations of mimicry,
warning coloration, or what not, without making them the objects
of careful experiment. As a matter of fact the meaning of colours
or markings in particular cases is often obscure, and it is better
to suspend one's judgment than to come to a hasty conclusion.

MIMICRY AMONG VERTEBRATES. — Wallace (in Darwinism)
cites the resemblance between Cuckoos and certain other Birds as
an instance of imperfect mimicry, and that between East Indian
Orioles and Friar Birds as a more perfect example of the same
phenomenon. Cuckoos are not well provided with the means of
defence, and it is remarkable that the Common Cuckoo is not
unlike a Sparrow-hawk in appearance and mode of flight, while
other species are known which are liable to be mistaken for
Drongo-shrikes, Starlings, and Pheasants respectively. All the
forms supposed to be mimicked are more powerful and less liable
to be attacked than the Cuckoos which resemble them.

3IO ANIMAL DEFENCES

The other case is much more remarkable. Among the Birds
inhabiting the Austro-Malay islands (i.e. the eastern half of the
East Indies) are weak Orioles and comparatively powerful Friar-
Birds or Honey-eaters, which are provided with strong beaks
and claws. Each of the large islands possesses its own peculiar

Fig. 487. — A Friar-Bird (Philemon Timor laoensis], right, mimicked by an Oriole (Oriolus decipiens}, left

species of Friar- Bird, and also a species of Oriole corresponding to
it in appearance and thereby enjoying, it is believed, a less worried
existence than would otherwise be the case. Forbes discovered
such a pair in Timor-laut (Oriolus decipiens and Philemon Timor-
laoensis) (fig. 487), and (in A Naturalist's Wanderings in the
Eastern Archipelago] speaks of them as follows: — " For some
time I was quite puzzled by the difference of behaviour of certain
individuals in flocks of these Birds [i.e. Honey-eaters] on the
trees. Only after the closest comparison of the dead Birds in
my hand was the enigma solved by my perceiving that the Birds
were distinct species of widely -removed families, and I learned

BODILY CHARACTERISTICS— CONSPICUOUSNESS 311

later that I had obtained new examples of that most curious case
of mimicry first detected (among Birds) by Mr. Wallace, where
an Oriole constantly derives protection from its foes by acquiring
the dress of a Bird always of the same powerful and gregarious
Honey -eaters. . . . When my collection was laid out for de-
scription by Dr. Sclater, the Oriole's and the Honey- eater's
dress were so strikingly similar that the sharp eye of that dis-
tinguished ornithologist was deceived, and the two birds were
described by him as the same species."

Among Reptiles, Poisonous Snakes are so universally feared
by other animals that we should naturally expect them to be
mimicked by harmless forms. As in the case of Birds, the
imitation may be either of a general kind or carried out into
detail. The former is exemplified by a number of non-poisonous
Snakes which, when threatened or attacked, behave as if they
were venomous. The same thing is true for the harmless Blind-
worm, which is not a snake at all, but a snake-like Lizard.

Probably the best example of Poisonous Serpents exhibiting
warning colours copied by innocuous species is afforded by the
Coral Snakes of South America and the West Indies (see p. 303).
Different species of these are copied with extreme accuracy
by harmless Snakes belonging to several genera, and a par-
ticular poisonous species may have more than one kind of
imitator.

MIMICRY AMONG INSECTS. — The phenomena of mimicry are
better illustrated by Insects than any other animals, and indeed
the matter was first placed on a scientific basis by the researches
of Bates upon the Butterflies of the Amazon valley. It will there-
fore be convenient to make Butterflies and Moths our point of
departure. There are three sub -families of the former which
have their head-quarters in the tropics, and are possessed of
properties which cause them to be avoided by insectivorous
animals. The technical names of these three groups are Danaids,
Acraeids, and Heliconids, the members of all three being distin-
guished by colours and patterns that are regarded as having a
warning meaning.

Heliconids. — This sub-family is limited to and very charac-
teristic of tropical America, and it is the one specially studied
by Bates as mentioned above. He showed that a considerable
number of the included species are copied in a very faithful

3i2 ANIMAL DEFENCES

manner by palatable Butterflies belonging to several other families,
and also by certain Moths. Many of the mimicking species belong
to the Pieridae, which includes the common white Butterflies of
our fields and gardens, and these are naturally so unlike the
forms they mimic that Wallace remarks (in Darwinism): — " These
differences are as large and important as those between pigs and
sheep, or between swallows and sparrows; while English entomo-
logists will best understand the case by supposing that a species
of Pieris in this country was coloured and shaped like a small
tortoise-shell, while another species on the Continent was equally
like a Camberwell beauty — so like in both cases as to be mistaken
when on the wing, and the difference only to be detected by close
examination ".

The Danaids are found in the warmer parts both of the Old
and New Worlds, though they are most abundant in the tropical
parts of Asia. Their various species are mimicked to a very
large extent, and one of these, the Black - veined Brown or
Monarch (Anosia erippus), is interesting as an American form
which has extended its range to this country. In its original
home it is imitated by a harmless Butterfly (Limenitis misippus)
belonging to another family and closely allied to one of our
native forms, the White Admiral (Limenitis sibylla).

It is evidently more important for the well-being of the race
that the female should be protected than the male, and it is a
fact that only the female Butterfly in some mimicking species
assumes the garb of an unpalatable species living in the same
area. The difference between the two sexes in such cases is so
great that they are liable to be mistaken for distinct species.
But this is quite a simple matter by comparison with a case
described in detail by Trimen, in which the females of a South
African Swallow - tail Butterfly (Papilio merope] are of three
different sorts, unlike the male and unlike one another. The
reason for this is, that each of the three kinds mimics a distinct
species of Danais (D. echeria, D. niavius, and D. chrysippus).
Comparison with a closely-allied Swallow-tail (Papilio meriones)
from Madagascar gives us an idea of what the female of P. merope
was originally like. In this case there is no question of mimicry,
and the two sexes are very similar.

Acr&ids. — The members of the third Butterfly group notorious
for inedibility, i.e. the Acraeids, is found chiefly in tropical Africa,

ANIMAL COLORATION

BODILY CHARACTERISTICS— CONSPICUOUSNESS 313

where the various species are mimicked by a number of edible
Butterflies belonging to other families.

Mimicking Moths. — The three groups of Lepidoptera so far
mentioned are by no means the only ones which are more or
less endowed with noxious properties, as a result of which they
have been the subject of imitation by their cousins. Cases among
Moths are known even among British forms, the most familiar
example being that of the White Ermine Moth (Spilosoma
menthastri), which is copied by the female of the Muslin Moth
(Diaphora mendica). The colour is white, spotted with black,
and the species imitated is known to be unpopular among insect-
eating animals.

Clear-wing Moths. — Some of the adult Lepidoptera imitate
members of other orders better endowed than they are with
means of defence. A very good instance of this is afforded by
the " clear- wing" Moths, so-called because the wing-membranes
have to a large extent lost the covering of scales which is so
characteristic of the lepidopterous order. Our two native species
of Hornet Clear- wing (Trochilium apiformis and T. crabroni-
formis), for example, look very like Hornets as a result of this
special character, with accompanying modifications of the pro-
portions, colours, and markings of the body. It is even stated
that when caught they make a pretence of stinging. That this
is really a case of mimicry is, however, doubted by some natural-
ists, as, e.g., by Sharp, who, speaking of the Clear- wing family (in
The Cambridge Natural History), says: — "Some of the species
have a certain resemblance to Hymenoptera, which is probably in
most, if not in all, cases merely incidental ".

Mimicking Caterpillars. — The larvae or caterpillars of a num-
ber of Butterflies and Moths would appear also to be protected
by mimetic devices, including more particularly the assumption,
when frightened, of alarming or warning attitudes of deceptive
nature. A remarkable case is that of the caterpillar of the
Lobster Moth (Stauropus fagi) (fig. 488), which, under ordinary
circumstances, is more or less protected by its resemblance in
colour and form to a distorted and withered leaf. When alarmed,
however, it rears up the ends of its body and vibrates, presenting
a sort of bogey-like aspect, which may be described as half spider-
like with the other half bug-like. Another remarkable caterpillar
is that of the Puss- Moth (Cerura vinula), which, when frightened,

3I4 ANIMAL DEFENCES

assumes a comic appearance which has been compared to that
of a small reptile, the front end being broadened out into a surface
looking like the caricature of a face with red margin and dark
eyes, while from the hinder end two pink whips are shot out,
these being specializations of the last pair of larval legs (pro-
legs). A gland from which an irritant acid fluid can be squirted
out opens on the lower part of the apparent " face ". Some large

Fig. 488.— Caterpillars of the Lobster Moth (Stauropusfagi)

caterpillars possess eye-like markings which give them a remote
resemblance to snakes, as, for example, in the case of those belong-
ing to the Large and Small Elephant Hawk-Moths (Charocampctr
elpenor and C. porcellus).

Mimicking Flies. — A good example of mimicry among two-
winged flies (Diptera) is that of the Drone- Fly (Eristalis tenax)
(see fig. 390), which closely resembles a bee in appearance and
movements, besides which its loud buzz is suggestive of the more
powerful insect it is generally supposed to imitate.

Mimicking Beetles. — Beetles (Coleoptera) present a variety
of cases usually believed to illustrate mimicry. As in the case
of insects belonging to other orders, a number of beetles would
appear to ward off attack by taking on the appearance of wasps
or bees. Poulton instances a British form (Clytus arietis] which
looks and moves very like a wasp, although the wings of the
latter are not imitated. This, however, is not so remarkable as

BODILY CHARACTERISTICS— CONSPICUOUSNESS 315

a Bornean beetle (Coloborkombus fasciatipennis], which is ex-
tremely like a large black wasp (Mygnimia aviculus] from the
same region. Beetles usually keep their transparent hind-wings
folded up except during flight, and covered over by the hard
fore-wings, which constitute wing-cases (elytra). But here the
latter are reduced to inconspicuous scales, while the former are
kept expanded, in which condition they closely resemble in form
and colouring the wings of the wasp.

Tiger- Beetles are avoided on account of their ferocity, many
Weevils because they are hard and indigestible, while other species
of Coleoptera are protected by stink-glands. All these protected
groups are mimicked by their weaker brethren.

Mimicking Grasshoppers and Crickets. — The cockroach and
grasshopper order (Orthoptera) is rich in illustrations of mimicry.
Semper (in Animal Life), for example, cites two Philippine
Grasshoppers, one of which (Scepastus pachyrhyncoides) pre-
sents a deceptive resemblance to a hard weevil (Apocyrtus),
while the other is a respectable imitation (Pkoraspis) of an un-
palatable lady-bird beetle (Coccinella). A Cricket (Condylodeira
tricondyloides] from the same islands is wonderfully like a tiger-
beetle (species of Tricondyla) found in the same part of the world.

Mimicking Orthoptera. — Some of the Orthoptera when
alarmed display eye-like markings, reminding one of the cater-
pillars already described. In the Praying Mantis (Mantis re-
ligiosa) and other members of the same family such markings
are found on the inner sides of the thighs (femora) of the
fore-legs, which members are usually forwardly-directed, ready
to catch insect prey. An interesting new theory regarding such
markings is quoted by Annandale (in the paper already men-
tioned), who says: — "I do not know that a function has ever
been assigned to marks situated in this position except by the
Russian naturalist Porschinsky. ... [He] has a theory that all
eye-like markings on insects represent glands, which may be
imagined to secrete a noxious fluid. He supposes that such
markings simulate the liquid which has issued forth, with the
blue sky or some other object reflected in it, and points out
that the display of such spots is sometimes accompanied by a
sound which might be taken to imitate liquid hissing out of
a narrow opening, such as the duct of a gland. Mantis religiosa
is one of his examples. He says that there is a large blue ' eye ',

3i6 ANIMAL DEFENCES

ringed with black, on the inner surface of the femur of the fore-
limb in this species; and that the eye is concealed when the
mantis is at rest, because the two limbs are held folded together
in front of the body. ' But when danger threatens ', to quote his
own words, ' the praying mantis assumes a very peculiar and
interesting attitude, which, so far as I know, was first described
by Goureau. The long and narrow pro-thorax assumes a ver-
tical position, so that the body is supported only by its two
pairs of hind-legs. Under these circumstances the insect widely
separates the front pair of legs, giving to its long femora a
horizontal position, so that the distal ends of them are directed
on opposite sides. In this way the eye-spots, which are situated
at their bases, stand out conspicuously and are most obvious
owing to their colour. The tibiae of the front pair of legs are
directed vertically upwards. At the same time the insect lifts
up its tegmina [i.e. wing-covers] and unrolls its wings, giving
them a horizontal position, and it begins quickly to raise and
lower its abdomen, which, rubbing against the posterior edge of
the wings at the same time as these continual movements, pro-
duces a sound. The mantis can produce the latter artificially
by rubbing its wings against some extraneous object." All
this is very speculative, but it would appear to be, at any rate,
an instance of a spurious warning.

Mimicking Plant-Bugs. — The last case of mimicry among
insects to be described here is in some ways the most remark-
able of all. An account has elsewhere been given (see p. 208)
of the leaf-cutting ants of South America, which cut off and
carry home pieces of leaf. W. L. Sclater has pointed out
that, associated with these ants, there is a kind of plant-bug
which, in colour and shape, actually resembles such an ant,
together with the piece of leaf it is carrying (fig. 422).

MIMICRY AMONG SPIDERS. — Spiders often form a favourite
diet with insectivorous birds, but it would appear that in various
parts of the world some kinds of these creatures obtain a little
peace and quietness by pretending to be ants. A very instruc-
tive North American species (Synageles picata] of the kind is de-
scribed by Peckam. Not only does this form resemble an ant
in general form, but also in gait and general behaviour; while its
second pair of legs are held up like feelers, the remaining three
pairs masquerading as the six legs of an insect.

BODILY CHARACTERISTICS— CONSPICUOUSNESS 317

A sufficient number of examples illustrating special bodily
characteristics have now been given, though it must not be
supposed to be a phenomenon limited to the groups which have
been drawn upon. We pass on to protective measures constituted
by certain habits not necessarily associated with peculiarities of
colour.

CHAPTER XXVII
ANIMAL DEFENCES— SPECIAL PRECAUTIONARY HABITS

We have so far considered PRECAUTIONARY MEASURES whereby
enemies are warded off, so far as they depend upon bodily charac-
teristics producing inconspicuousness or studied conspicuousness,
but certain PRECAUTIONARY HABITS of quite different kind are
equally important, and deserve a little attention. A great deal
of what might be said has of necessity been anticipated, for the
form and structure of animals can only be properly understood
when considered along with habits and surroundings.

It has already been pointed out that animals are most ex=
posed to the attacks of enemies when they are on the move.
Excluding forms such as sponges, corals, sea-lilies, &c., we may
say that movements are more or less necessary for the purpose
of obtaining food. The risk thus incurred, however, may be
reduced by feeding at suitable times and in suitable places, while
safety between whiles may be gained by the existence of dwellings,
retreats, and temporary resting-places.

FEEDING AT FAVOURABLE TIMES— NOCTURNAL
ANIMALS. (Fig. 489)

This subject requires working out, so that only a few points
can be here presented for consideration. A very large number
of those animals most exposed to attack turn night into day,
and, in certain cases at any rate, the nocturnal habit may have
been acquired as a protective measure. It is necessary here,
however, to be guarded in statement, since many factors have
to be considered, and consequently every case must be weighed
carefully. It is not unlikely, for example, that the nocturnal habit
may be a very ancient one, having no special protective signifi-
cance, and that its continuance in many recent forms is due to

318

Fig. 489. — Heads of Nocturnal Animak

i, Hawk-Owl (Sumia ulula}; 2, Owl-Parrot (Stringops Jiabroptiius] ; 3, Slow Lemur ( Nycticelus tardigradus] •
4, Spectre -Tarsier (Tarsius spectrum}; 5, Night -Monkey (Nyctipithecus trivirgatus] ; 6, Egyptian Jerboa
(Dipns jaculus) • 7, Wall-Gecko ( Tarentola Mauritanica) ; 8, Flying Frog (Rhacopfwrus Reintwardti) ; 9,
Phosphorescent Sardine (Scopelus engraulis).

319

32o ANIMAL DEFENCES

the action of heredity through a long series of ages. In very
hot climates, too, the question of temperature has to be taken
into consideration. And even where we may suppose the habit
to be protective it can only be partially effectual, for many
predaceous animals are notoriously nocturnal, and some of them,
at any rate, have become so because their prey took to feeding
at night as a protective measure. We may feel sure that every
new life-preserving device that has been evolved among animals
which are preyed upon results in some counter-move among the
aggressors.

Some few monkeys are nocturnal in habit, and among these
may be particularly mentioned the Owl-faced Night -Monkeys
(species of Nyctipithecus], widely distributed in South America.
The time at which these small animals feed is indicated, as in
many other cases, by their large eyes, around which the hair
is arranged in a radiating manner, the two features combined
giving a very owl-like appearance. The chief enemies of the
inhabitants of South American forests are climbing snakes and
carnivores, and probably the attacks of these are to some ex-
tent avoided by the practice of feeding at night. But it must
be borne in mind that these night-monkeys themselves feed upon
insects and birds, which are more easy to surprise in the dark, so
that in this case the nocturnal habit would appear to serve a
double purpose.

Primitive groups of animals often include a number of noc-
turnal members, and this is well seen in the order of Lemurs,
many of the smaller kinds of which are endowed with large
staring eyes. Examples are the Aye-aye (Chiromys) of Mada-
gascar, and the Pottos (Perodicticus and Arctocebus) and Galagos
(Otolicnus) of Continental Africa, together with the Loris (Nyc-
ticebus) and Spectre-Tarsiers (Tarsius] of the East Indies. As,
however, most of these live largely or mainly upon insects and
small vertebrates, the night-feeding habit is probably useful in
two ways, as in the case of the owl-faced night-monkeys already
described. If, as has been asserted, the West African Galagos
subsist only upon fruit and gum, the nocturnal arrangement may
possibly be purely protective.

Bats are perhaps the most thorough-going night-forms among
Mammals, but as the great majority prey at dusk upon insects
which fly at that time, the habit is probably in relation to this.

SPECIAL PRECAUTIONARY HABITS 321

Even the Fruit- Bats (Pteropus) partly affect an animal diet,
devouring various small vertebrates, from fishes to mammals.
The large majority of the Insectivora are night-feeders, pro-
bably because their prey is more easily secured at that time
than for the sake of protection.

So powerful a creature as the Elephant seems at first sight
to require no special means of protection, and its nocturnal
habits are no doubt largely due to its dislike of heat and desire
to avoid the unpleasant attention of flies. No animal, however,
is more persecuted by the arch-enemy man, who is more par-
ticularly given to hunting during the day, so that nocturnal forms
have the best chance of avoiding him.

Many of the Ungulates feed at night, partly, it would seem,
for the sake of thereby lessening the chance of attack, but also
in order to avoid heat and flies. The habit is particularly well
developed in the case of certain forms which affect damp surround-
ings and wooded country, of which good examples are afforded
by Hippopotami, Tapirs, Swine, some kinds of Deer, and Cape
Buffaloes. Regarding the Hippopotamus, Vogt (in The Natural
History of Animals) says: — "The hippopotamus is on the whole
a nocturnal animal, and where it has made acquaintance with
firearms leaves the water only by night, or if by day, only to
bask in the sun on sand-banks and islands out of the range
of bullets ". Regarding the Cape Buffalo the same writer re-
marks : — " It is fond of plains and marshy forests, and delights
to remain the whole day buried in mud up to the shoulders in
order to protect itself against insects by which it is infested, and
from which it is partly delivered by birds that settle on its back ".

The Gnawers or Rodents constitute a very large and widely-
distributed order of Mammals, living mainly on vegetable food,
mostly of small size and ill-provided with the means of defence,
on which account they are particularly liable to the attacks of
predaceous forms. Taking all these facts into consideration, we
shall probably not be far wrong in attributing a protective func-
tion to the nocturnal habits by which most of them are charac-
terized. Rats and Mice may be regarded as a case in point, and
the ordinary House-mouse (Mus musculus) in particular would
have but a poor chance of existence if all its depredations were
carried on during the day. Another feature of interest in this case
is that it presents a pretty clear case of a protective habit which

VOL. II. 53

322 ANIMAL DEFENCES

has been rendered less useful because it has also been adopted
by certain natural enemies, i.e. Owls, which are among the most
persistent foes of small rodents. In these birds the nocturnal
habit has almost certainly been acquired for aggressive pur-
poses. It is interesting to note that some members of the order
possess the large eyes characteristic of thorough -going nocturnal
species. This is the case, e.g., with the Egyptian Jerboa (Dipus
^Egyptiacus}.

The Edentates, like many other ancient and primitive groups,
are markedly nocturnal, and the leaf-eating Sloths in particular
probably derive a good deal of protection from the habit. The
same remarks apply to the more defenceless vegetable -feeders
among the Marsupials, such as the Phalangers, Koalas, and
Wombats. The more powerful Kangaroos, on the other hand,
feed by day, while the rapacious Dasyures and the Tasmanian
Wolf prey at night on weaker animals.

The feeble members of the lowest group of Mammals, the
Monotremes, are nocturnal in habit, the Spiny Ant-eaters entirely,
and the Duck-billed Platypus largely so.

Birds are essentially a day-loving class, and when nocturnal
habits have been acquired, as in owls and night-jars, they chiefly
have reference to the nature of the prey. In some few cases,
however, the night-feeding practice would appear to have a pro-
tective meaning, as, e.g., in two New Zealand birds, the Owl- Parrot
or Kakapo (Stringops habroptilus] and the Kiwi (Apteryx). The
former feeds chiefly on vegetable matter, and being practically
devoid of flying powers is singularly defenceless; its time of
activity might be divined by the owl-like character of its head.
The Kiwi is the smallest and most defenceless of the running birds.

Among Reptiles, — Crocodiles and large Snakes, such as
Pythons, are respectively largely and entirely nocturnal, but pro-
tection is not the end to be attained. A case, however, of which
the meaning is probably partly protective, is afforded by most of
the species of the little climbing lizards known as Geckos, of which
large eyes are characteristic, and these organs exhibit the vertical
pupils often found among nocturnal forms. The specialization
even here, however, has no doubt as much reference to the pursuit
of certain sorts of insects under favourable conditions as to protec-
tion ; indeed, the former may be the chief object of the habit.

Although Amphibians constitute a group which is predomi-

SPECIAL PRECAUTIONARY HABITS 323

natingly nocturnal, this habit, though perhaps partly protective,
is primarily due to the fact that a large amount of moisture is
essential to these creatures, for any great amount of evaporation
from their skin (which helps in breathing) would be deleterious,
if not fatal. Large eyes are often possessed by them, and the
Flying Frog (species of Rhacophorus) are particularly well en-
dowed in this respect.

Many Fishes are of nocturnal habit, and there can be little
doubt that in some cases protection is one of the ends thus
attained. But, precisely as in land -vertebrates, the value of
this arrangement is much reduced by the fact that predaceous
forms may have the same habit. As fishes commonly discover
their food by means of touch and smell, darkness is not the
same hindrance to feeding that might be anticipated.

Many Invertebrates are nocturnal, but our knowledge of their
habits is too scanty in most cases to permit of more than some-
what vague conjecture. It is highly probable, however, that the
practice of feeding at night exemplified by such insects as Cock-
roaches, Crickets, and Moths acts more or less as a protection.
Nor is it impossible that this may be one of the ends served by
the phosphorescence of certain insects. This phenomenon is
perhaps most strikingly seen among the Fire- Flies, which con-
stitute a family (Lampyrida) of tropical beetles.

Many marine invertebrates, belonging to widely -differing
groups, are more active by night than day. Hosts of creatures
which during the latter are submerged to some depth rise to
the surface when the sun goes down, especially when the weather
is calm. It is not known how far this habit has to do with pro-
tection, nor is the use of the phosphorescence characteristic of
many such forms fully understood.

We have now seen that certain animals gain a measure of
protection from their foes by feeding during the night, but as the
same end is attained in other cases by feeding during the day,
it may be in such a way as to court rather than avoid observa-
tion. Space forbids the mention of more than a few typical
instances.

FEEDING AT FAVOURABLE TIMES— DIURNAL ANIMALS

Many herbivorous animals possessed of rapid means of pro-
gression, and endowed with keen senses by which the approach

324

ANIMAL DEFENCES

of enemies may be perceived, feed boldly during the day, relying
upon their powers of rapid retreat in the case of attack. This
is particularly noticeable in those species which obtain their food
in places where a wide outlook over the surrounding country is
obtainable. Wild Horses and their allies, Giraffes, Antelopes,
Goats, Sheep, Kangaroos, and Ostriches, will serve as examples.

It is also obvious that species exhibiting warning coloration,
and mimicking animals, are specially equipped to secure their
protection during the day, provided that the current interpretation
of these phenomena rests on a firm basis. The same thing is
probably very largely true of protective resemblance, whether this
is special or general, as after dusk no useful end can be served by
such arrangements, except perhaps on bright moonlight nights.

Having now discussed in a tentative fashion cases where some
amount of protection is gained by animals which are active at
certain Times, we come to instances where the Place of feeding
is useful in this respect.

ANIMALS PROTECTED BY FEEDING IN SUITABLE

PLACES

Under this heading may be, in the first place, included all
those forms which exhibit protective resemblance, whether general
or special. It is quite clear, for example, that a colour-scheme
which harmonizes with certain surroundings so as to result in
inconspicuousness, may be very conspicuous if viewed against
some other kind of background. Romanes (in Darwin and
After Darwin], in describing a particularly good instance of this,
says: — " . . . Hares and rabbits . . . instinctively crouch upon
those surfaces the colours of which they resemble; and I have
often remarked that if, on account of any individual peculiarity
of coloration, the animal is not able thus to secure concealment,
it nevertheless exhibits the instinct of crouching which is of
benefit to all its kind, although, from the accident of its own
abnormal colouring, this instinct is then actually detrimental to
the animal itself. For example, every sportsman must have
noticed that the somewhat rare melanic [i.e. black] variety of
the common rabbit will crouch as steadily as the normal brownish-
grey type, notwithstanding that, owing to its abnormal colour, a
' nigger-rabbit ' thus renders itself the most conspicuous object in
the landscape."

SPECIAL PRECAUTIONARY HABITS 325

As mentioned in a preceding paragraph, certain forms endowed
with the power of rapid movement are comparatively safe when
feeding in places from which a wide outlook is obtainable, and
this may be flat or undulating country, such as is favoured by
giraffes, many antelopes, wild horses, kangaroos, and ostriches,
or it may be of the mountainous rocky character affected by wild
sheep and goats, conies, baboons, and many other creatures.
Even where the powers of locomotion are not very extraordinary,
open places afford comparatively safe feeding-ground if a suitable
retreat is close by. Young Rabbits, for example, commonly feed
quite close to the mouth of their burrow, into which they imme-
diately disappear on the least alarm. Similarly arboreal forms
which feed to some extent on the ground, as is the case with a
number of Old- World monkeys, are pretty safe so long as trees
are at hand. This naturally suggests the next case to be con-
sidered, i.e. the arboreal or tree-inhabiting habit as a means of
protection.

ARBOREAL ANIMALS. — Several causes have had to do with the
evolution of climbing animals, and the remarks already made in
reference to the nocturnal habit (see p. 318) are equally applicable
here. Such an infinite variety of animals exist, and they increase
so rapidly, that the struggle for existence is exceedingly keen, and
every possible kind of food is liable to be commandeered. And
since (see p. 164) practically all animals are dependent on plants
either directly or indirectly, it would be extraordinary if woods
and forests had not attracted a large population, which has be-
come more or less specialized in accordance with the exigencies
of an arboreal life. The following remarks made by Bates (in
The Naturalist on the Amazons], in reference to the animal
population of the virgin forests of South America, forcibly illus-
trate this point. After speaking of certain climbing plants, he
says: — " The number and variety of climbing trees in the Amazon
forests are interesting, taken in connection with the fact of the
very general tendency of the animals also to become climbers.
All the Amazonian — and in fact all South American — monkeys
are climbers. There is no group answering to the baboons of
the Old World, which live on the ground. The Gallinaceous
birds of the country, the representatives of the fowls and pheasants
of Asia and Africa, are all adapted by the position of the toes to
perch on trees, and it is only on trees, at a great height, that they

326 ANIMAL DEFENCES

are to be seen. A genus of Plantigrade Carnivora, allied to the
bears (Cercoleptes\ found only in the Amazonian forests, is entirely
arboreal, and has a long flexible tail like that of certain monkeys.
Many other similar instances could be enumerated, but I will
mention only the Geodephaga, or carnivorous ground beetles, a
great proportion of whose genera and species in these forest
regions are, by the structure of their feet, fitted to live exclu-
sively on the branches and leaves of trees. . . . The largest and
most interesting portion of the Brazilian mammal fauna is arboreal
in its habits; this feature of the animal denizens of these forests
I have already alluded to. The most intensely arboreal animals
in the world are the South American monkeys of the family
Cebidse, many of which have a fifth hand for climbing in their
prehensile tails, adapted for this function by their strong muscular
development, and the naked palms under their tips. This seems
to teach us that the South American fauna has been slowly
adapted to a forest life, and therefore that extensive forests must
have always existed since the region was first peopled by mam-
malia." Enough has been said to show that food-supply alone
must have had a great deal to do with the evolution of arboreal
forms, and it would be easy to draw up a number of cases on the
model of The Ho^lse that Jack Built to illustrate chains of causes
and effects that have arisen as a result. Trees have produced
fruits and seeds, fruits and seeds have led to the evolution of
fruit- and seed-eating monkeys, &c., fruit- and seed-eating monkeys,
&c., have led to the specialization of climbing carnivores. Simi-
larly the wood and bark of trees have afforded nutriment to
various insects, and this has resulted in the evolution of wood-
peckers and other arboreal insectivorous forms.

All this may be the truth, and nothing but the truth, but it is
by no means the whole truth. It is practically certain that some
animals have taken to feed among trees, not merely for the sake
of the aliment there present, but also in order to avoid enemies
living upon the ground. In such cases the arboreal habit is partly
a protective measure. Why, for example, should the carnivorous
ground-beetles of the Amazon forests have taken to live among
the trees, as stated by Bates in the extract given above? A
possible solution to this is given by the same zoologist elsewhere.
" It is vain to look for the Geodephaga, or carnivorous beetles,
under stones, or anywhere, indeed, in open, sunny places. The

SPECIAL PRECAUTIONARY HABITS 327

terrestrial forms of this interesting family, which abound in
England and temperate countries generally, are scarce in the
neighbourhood of Para; in fact I only met with four or five
species. On the other hand, the purely arboreal kinds were
rather numerous. The contrary of this happens in northern
latitudes, where the great majority of the species and genera
are exclusively terrestrial. . . . The remarkable scarcity of
ground -beetles is doubtless attributable to the number of ants
and termites which people every inch of surface in all shady
places, and which would most likely destroy the larvae of
Coleoptera."

Some of the members of certain ancient groups now on the
decline have taken to an arboreal life, trees affording them a
refuge which at the same time yields an abundance of food.
Such, for example, are the defenceless leaf-eating Sloths, crea-
tures of comparatively small size, which are remarkably special-
ized in structure to fit them for their life among the trees of the
South American forests. The explanation here suggested is sup-
ported by the fact that tree -dwelling sloths are a young group,
geologically speaking, and find their nearest allies among extinct
forms in the Ground- Sloths, which, though they appear to have
lived on foliage, were quite unable to climb. Some of these
creatures attained a very large size, Megatherium, for example,
rivalling the elephant in that respect. The Neomylodon of Pata-
gonia, perhaps even yet living, is, or was, a similar kind of
animal.

PARACHUTE ANIMALS. — Certain climbing forms have developed
folds of skin converting them into what may be termed " parachute
animals", an arrangement which facilitates progress from one tree
to another, and is protective in so far as it may be supposed to
facilitate escape from enemies. Such among mammals are Flying
Squirrels, while lizards present the case of Flying Dragons, and
Flying Frogs are found among amphibians. From this kind of
apology for flight we naturally pass to flying animals proper.

FLYING ANIMALS. — The geological record proves that Insects
led the way in the conquest of the air, thereby vastly facilitating
their progress from place to place in pursuit of prey, and affording
more or less protection by thwarting many of the attacks of wing-
less enemies. And it is noteworthy that the oldest known insects
find their nearest allies among recent Orthoptera of the cockroach

328 ANIMAL DEFENCES

kind, and were presumably vegetarian. The evolution of flying
insects made the realm of air a new food-producing territory, and
this may have been one of the factors helping to bring about the
evolution of Birds, Flying Reptiles (long since extinct), and Bats.
Be that as it may, the flying habit in recent birds and bats affords
a certain amount of protection, though the device is so ancient
that its efficacy in this respect has been largely discounted by the
appearance of rapacious types with powers of flight. Insects, the
oldest fliers, have naturally suffered most from this cause, for they
prey upon one another, and are mercilessly thinned out by birds
and bats. Birds, which come next in order of antiquity, prey
upon one another, and it would appear that bats, the youngest
(so far as our knowledge goes) of flying groups, enjoy the largest
share of protection from their power of progression through the
air. It would be rash to even conjecture why flying reptiles
should have become extinct. Perhaps the competition with birds
became too keen; at any rate the experiment turned out a failure,
though in justice to their class it should be remembered that birds
are probably of reptilian stock, which may therefore be said to
have made two series of attempts at flight, conducted on different
lines, and one of which has been crowned with success.

UNDERGROUND ANIMALS. — We have now seen how certain
forms of life have withdrawn themselves from the keen com-
petition which takes place upon the surface of the ground, and
gained more or less protection, while at the same time improving
their chances of getting sufficient food, by adopting an arboreal
or an aerial habit. Another line has been struck out by creatures
which have taken to live entirely underground. The subterranean
parts of plants and the organic matter contained in earth have
offered food to vegetarian forms, and these again have led to the
evolution of carnivorous species suited for progression below the
surface of the soil. This way of life has proved of advantage to
many sorts of animal, since it affords protection as well as food.

Among vegetarian forms of the kind may be more particularly
mentioned Earth- Worms, certain adult Insects (e.g. Mole-Crickets),
and many Insect larvae (e.g. Cockchafer Grubs). Among carni-
vorous forms most specialized in accordance with underground
habits are found members of widely -different groups. As to
Mammals, we have, for instance, the Moles, belonging to the
large and primitive order of Insectivora, and the Pouched- Mole

SPECIAL PRECAUTIONARY HABITS 329

(Notoryctes) of Australian deserts, belonging to the still lower
order of Marsupialia. Such Reptiles as the small degenerate
Burrowing- Snakes (Typhlopida) and certain Snake-like Lizards
(Ampkisbanida) are further examples, and still another instance
is afforded by the curious tropical Caecilians, constituting an order
in >Amphibia which present a superficial resemblance to Serpents.
It may be noted here, too, that a rich fauna inhabits certain caves.

This section may appropriately be concluded by a few remarks
on protective measures as to place of feeding which have been
adopted by aquatic forms.

CERTAIN MARINE ANIMALS WHICH HAVE GAINED PROTECTION
BY MIGRATING TO NEW FEEDING-GROUNDS. — There is good reason
for believing that the sea is the original home of life, and from
very ancient times its shallow waters up to high-tide mark have
been the scene of very keen competition for food. By migration
from shallow water in different directions certain forms tapped
fresh sources of food-supply, and, for a time at any rate, succeeded
in reducing the toll levied upon them by predaceous forms. It
is extremely probable that many Land-Animals originally took
origin from the fauna existing between tide-marks, the conditions
there being such as to afford a preparatory training for terrestrial
existence pure and simple. Backboned Land-Animals most likely
sprang from freshwater forms (see next paragraph), and these
again claim marine ancestry. The land, once peopled, soon pre-
sented a struggle for existence as keen as that obtaining in the
sea, and in the ways already indicated the pressure was partially
relieved. Another particularly interesting case here deserves
mention, i.e. that of various forms which, after becoming thorough-
going land animals, returned to the original home of life, and
underwent a second series of specializations fitting them for a
new sort of marine existence. In this way two of the orders
of Mammals have been evolved: Cetacea, including Whales,
Porpoises, &c. ; and Sirenia, comprising Manatees and Dugongs.
Among the Carnivora, too, we have the Pinnipedia (Walruses,
Sea- Lions, and Seals), which live more in the sea than out of
it. A similar policy has been pursued by members of some
other classes of land Vertebrates. Penguins, for example, have
given up flight, and spend a large part of their lives in the sea,
using their modified wings as paddles. Several groups of extinct
Reptiles appear to have lived entirely in the sea; why they died

33o ANIMAL DEFENCES

out it is difficult to understand. One reptilian experiment in this
direction, however, has been more successful, for Marine Snakes
thrive in the Indian Ocean. Among Invertebrates the lung-
breathing Snails and Slugs (Pulmonata) are very much special-
ized to fit them for a life on land, yet certain slugs (species of
Oncidium^ &c.) have taken to a shore -life, though they can
scarcely be called marine. Insects are perhaps the most charac-
teristic of land-animals, yet a few of them are found living on
the surface of the sea. Certain allies of the Earth- Worm have
been found burrowing along the shore.

RIVERS AND LAKES AS A HAVEN OF REFUGE. — The waters of
the land have afforded protection to a number of hard-pressed
marine forms, estuarine conditions affording a sort of half-way
house. Fishes afford the most striking illustration, for all of those
inhabiting rivers and lakes have probably sprung from a marine
stock. At the present time, for example, Lung- Fishes (Dipnoi)
are limited to some of the rivers of Africa, South America, and
Australia, though originally the group to which they belong
included only marine species, as shown by geological evidence.
All the marine species have now become extinct, having been
unable to cope with the competition offered by other forms, but
those which have taken to fresh water have so far been able
to hold their own. Amphibians appear to have been evolved
from ancestors resembling Lung- Fishes in many ways, while
Reptiles, Birds, and Mammals probably sprang from an am-
phibian stock.

FLOATING AQUATIC FORMS (Plankton). — The surface layers of
the sea swarm with animal life, and have very likely (though this
is not quite certain) received their population from shallower
water. The gulf-weed which floats on the Sargasso Sea in the
North Atlantic has quite a fauna of its own, and no doubt plants
took to floating life before animals, which were attracted by the
food-supply thus offered. There is reason to believe that in very
remote geological times far larger areas of the ocean surface were
covered by drifting sea-weed than is the case at present. Such
floating animals are also found in lakes.

DEEP-WATER LIFE (Benthos). — Profound ocean depths are
peopled by a large and strange collection of animal forms, many
of which seem to have been driven out of shallower water by
the force of competition. Otherwise they would most likely have

SPECIAL PRECAUTIONARY HABITS 331

become extinct, so that their taking to deep-sea life has proved
a protective measure. The nature of these forms and the con-
ditions under which they live will be discussed elsewhere. Lakes
also possess a deep-water fauna.

BURROWING MARINE FORMS. — As explained elsewhere (p. 249),
many marine Bivalve Molluscs provided with siphons are able
to feed and breathe when buried in sand or mud, the tips of the
siphons only projecting. This is a very effective protective measure,
especially as sense-organs are present, which enable their owner to
distinguish between degrees of light and darkness. As one result
of this, a passing shadow, often an indication of the presence of
enemies, causes the siphons to be drawn in. Other bivalves
burrow in stone or wood, and the same is true for the members
of some other groups. A large number of Marine Worms dwell
in mud or sand like their terrestrial brethren, and enjoy a certain
amount of protection as a result, though here again certain pre-
daceous forms have adopted the same mode of life with intentions
the reverse of benevolent.

CHAPTER XXVIII
ANIMAL DEFENCES— PASSIVE DEFENCE

A sketch having now been given of PRECAUTIONARY MEASURES,
we come to the various kinds of RESISTANCE, by which the attacks
of enemies when delivered are met, and, it may be, foiled.

It is clear that defence may be either PASSIVE or ACTIVE, and
the two cases are best considered separately.

PASSIVE DEFENCE

As in the case of Precautionary Measures, it may conduce to
clearness if we look at this matter from two different but closely-
connected standpoints: i.e. (i) Bodily Characteristics; (2) Special
Habits; (3) Fecundity, as a means of defending the species, also
requires consideration.

BODILY CHARACTERISTICS OF USE IN PASSIVE

DEFENCE

UNPALATABLENESS AND INDIGESTIBILITY. — In dealing with
warning coloration (see pp. 301-309), it has been pointed out
that unpalatable or indigestible forms are often distinguished
by colours and markings which are more striking than artistic.
Such animals, however, are liable to "experimental tasting" on
the part of inexperienced enemies, and though some of these,
if rapidly ejected on account of their objectionable properties,
may escape with their lives, many or most are not so fortunate.
Even these, however, contribute to the defence of the species,
as they assist in the education of enemies. At least this is the
current explanation, and, so far as some animals are concerned,
it is based on actual experiments, though whether these are
sufficiently numerous and extended to form a safe basis for
generalization has been doubted. However this may be, we
are on safe ground as regards those animals which are provided

PASSIVE DEFENCE

333

with defensive plates and spines which oppose considerable re-
sistance to attack.

ARMOURED ANIMALS. — Both plate-armour and spiny coverings
are found in many groups of the animal kingdom, the former
simply warding off teeth and claws, while the latter are calcu-
lated to inflict injury upon enemies coming to close quarters.
Several orders of Mammals possess arrangements of the kind.
Among Edentates,
for instance, Arma-
dilloes are pro-
tected by cuirasses
composed of bony
plates, and Pango-
lins by scale -ar-
mour. Among Ro
dents, the Porcu-
pines are clothed
with effective spines (sometimes barbed), as are Hedgehogs among
Insectivores, and Spiny Ant- Eaters among Monotremes.

Birds are provided with scale-armour on their legs, and their
feathers constitute a protective coating by which, no doubt, many

Fig. 490. — Diagram of varieties of Reptilian Armour, as seen in section
A, Granular scales. B, Flat scales or shields, c, Overlapping scales. D, The
same, with underlying bony plates (scutes), ft, Hard outer layer of epidermis;
s, deeper layer of epidermis; /; dermis; <?, bony plates.

Fig 491.— Nile Crocodile. Two scutes, covered by horny epidermal plates. From a photograph

bites and stings are prevented from taking effect upon the under-
lying skin, which is here unusually thin and delicate. Armoured
Reptiles are common (fig. 490), Crocodilians (fig. 491) and Chelo-
nians (turtles and tortoises) affording the best instances of protec-
tive plates, while one of the Australian Lizards (Moloch horridus]

334

ANIMAL DEFENCES

will serve as an illustration of a spiny covering. The defences of
tortoises and turtles have been already described (vol. i, p. 214),
but it may be repeated here that such an animal is, as it were, en-
closed in a firm box (fig. 492), mostly formed of an upper (carapace)

and a lower (plastron) shield.
Head, tail, and limbs can be
more or less drawn back
into the shelter thus af-
forded. The rear defences
are strengthened by an
ingenious device in the
Hinged Tortoises of Africa.
In these creatures the hin-
der part of the carapace is
connected with the region
in front of it by a sort of
transverse hinge formed by
an elastic ligament, so that
when tail and hind-legs are
drawn in, the movable part
of the shell closes like a
spring - door and protects
these parts from attack.

Armour is singularly deficient in recent Amphibians, but the
members of one extinct order of these creatures (Stegocephald)
were well off in this respect. A very extraordinary arrangement
is found in two living species of newt, i.e. the Spanish Newt
(Triton Waltli\ and another form (Tylototriton Anderson?)
native to the Loo-Choo Islands. These creatures possess long
sharp ribs, which sooner or later penetrate the skin. Afterwards,
as- Gadow remarks (in The Cambridge Natural History], " the
wounds heal up, the skin forming a neatly -finished -off hole
through which the spike projects, not as a formidable, but as a
sufficiently awkward, protective weapon". Protection of the kind
is well developed in many Fishes. Good examples of firm plates
are found in the Bony Pike (Lepidosteus), the Bichir of the Nile
(Polypterus\ and Coffer- Fishes (Ostracion). Short, strong spines
abound on the Thornback Ray (Raia clavatd], and the Globe-
Fishes (Diodon and Tetrodon) are covered by more numerous
and longer defences of the sort. In a large number of forms

Fig. 492. — Carapace and Plastron of a Tortoise, from which
the external horny plates have been removed

ne^-nez, Neural plates or broadened tops of vertebrae; co-cas,
costal or rib plates; nu, nuchal or neck plate; py1, #y2, pygidial
or tail plates; ml-m11, marginal plates; epp, hyop, hypp, xyp,
paired plates of plastron; enj>, unpaired plate of plastron.

PASSIVE DEFENCE

335

sharp spines are connected with some of the fins, and as it is the
dorsal ones which are best off in this respect, we may infer that
such fishes are most likely to be attacked from above by enemies.

Fig. 493. — Method of growth in Gastropod Shells. The three upper figures show upper surface, while the two
others are side views, a, b, c, d, e, represent successive additions to the shell.

Cat- Fishes (Ckimara and Callorhynchus], the Piked Dog- Fish
(Acantkias), and Perch-like Fishes may be taken as examples.

Many of the Mollusca are more or less perfectly enclosed in
shells, which receive successive additions as their owners increase
in size, and these are usually indicated by
lines of growth visible on the outer sur-
face. The shape of a Gastropod varies
in shape according to the way in which
additions are made (fig. 493), and the
same thing is also true of the shell of a
bivalve. A shell of the latter kind is
shown in section in fig. 494, and the
various stages of its growth are indicated
in a diagrammatic way.

Molluscan shells may be either of the
nature of plate-armour, or may be pro-
vided with sharp projecting spines.
Among the former are Pearly Nautilus
(Nautilus pompilius}, Garden Snail (Helix
aspersa), and Freshwater Mussels (Unio and Anodonta).

Among the spiny Sea- Snails may be mentioned species of

Fig. 494._Growth of Bivalve sheii as

336

ANIMAL DEFENCES

the genus Murex (fig. 495), also Scorpion -Shells (Pteroceras\
while Thorny Oysters (Spondylus) and the Spiny Cytherea
(Cytherea dione] are good illustrations of spiny bivalves. It is.
particularly interesting to notice in the last case that the spines are
massed at the posterior end of the shell, the part most exposed
to attack, for, although the animal may bury itself in the loose
covering of the sea-floor, the siphons projecting from its hinder-
end must be placed so that currents of water may enter and
leave (see p. 249) by the apertures there present, respectively

Fig. 495. — A spiny Sea-Snail (Murex]

Fig. 496.— Opercula of various Gastropods.

a, River Snail (Paludina); b, Whelk (Buccinum}; c, Murex;
d, Cyclostoma; e, Trochus;f, Nerita.

carrying food and oxygen inwards, and waste products of all sorts
outwards.

In many univalve Molluscs (Gastropoda] the shell is so roomy
that the animal can withdraw entirely into it. Such a creature
often possesses a special plate (operculum) by means of which it
can, as it were, close the door behind it (fig. 496). This plate
may be simply horny, as in a Whelk or Periwinkle, though not
infrequently it is composed of dense shelly material, as, for in-
stance, in Nerita and Neritina.

Some of the 'tween-tide Gastropods which live on rocks, such
as the Limpet (Patella] (p. 197), are descended from ancestral
forms which possessed a large spiral shell into which the body
could be withdrawn. The conical structure by which this has
been superseded is, however, a very efficient defence, for when
alarmed a Limpet holds on with great force by means of its
powerful foot, and pulls down the shell so that its edges touch
the surrounding rock. To dislodge the animal from its hold

PASSIVE DEFENCE 337

requires great force if it is once allowed to fix itself firmly
although, if taken unawares, a smart tap is generally sufficient.

Jointed-limbed Invertebrates (Arthropods] are invested in a
firm, horny external skeleton, sometimes thick enough to con-
stitute veritable plate-armour, and not infrequently garnished with
protective spines. Insects and Crustacea afford the best ex-
amples.

Insect Armour. — Beetles are often distinguished by the thick-
ness of their integuments, and, as previously mentioned (see p.

Fig. 497. — Caddis- Worm Tubes of various kinds (enlarged)

315), some of the weevils are so well oft; in this respect as to be
avoided by insectivorous birds. Certain tropical beetles (of the
genus Hispd] present a chevaux-de-frise of long, sharp-pointed
spines.

The larvae of Caddis- Flies, commonly known as " caddis-
worms ", make protective tubes by cementing together all sorts
of available substances (fig. 497).

Crustacean Armour. — To illustrate the smooth variety of
defensive armour-plating among Crustaceans, a better example
could hardly be chosen than the common Edible Crab (Cancer
pagurus]. Here the greater part of the body is enclosed in

VOL. II.

338 ANIMAL DEFENCES

an extremely thick carapace, strengthened by the deposit of
lime-salts, and the insignificant tail which, if left sticking out,
would constitute a weak place in the defences, is kept tucked
up out of harm's way into a depression on the under surface.
The walking-legs, too, can be folded up under the body, and
the great pincers brought together in front. The Northern
Stone-Crab (Lithodes maia) (fig. 498), pleasingly termed the

Fig. 498.— The Northern Stone-Crab (Lithodes maia), reduced

Devil-Crab by the Norwegians, and allied species from Japan
and elsewhere, are so thickly covered with sharp spines, pincers,
walking-legs and all, that they would undoubtedly prove rather
trying mouthfuls to even the most sharp-set of enemies. The
front part of* the body of the Rock- Lobster (Palinurus] (see vol. i,
p. 412) is also pretty well off in the matter of spines, though at
first sight it seems odd that its large tail should be quite smooth,
while part of the tail-fin is soft. The seeming anomaly is cleared
up when we remember that this animal is in the habit of watching
for prey with the smooth tail sheltered in a crevice, so that only
the thorny part of the body is exposed to the attacks of more
powerful creatures. Accommodation of armour to growth is
effected in this group by a method differing entirely from the
economical Molluscan plan. A Crustacean, in fact, undergoes a
series of " moults " until the adult maximum size is reached, and
these are very frequent in early life. At such times the entire
hard covering of the body is cast off, and also the firm lining of
the stomach (see vol. i, p. 407).

In free-living Bristle- Worms (Chatopoda) the bristles may be

PASSIVE DEFENCE 339

sufficiently well-developed and numerous to constitute a means
of defence, as, for instance, in the Sea-Mouse (Aphrodite aculeata)
(p. 147) and the Porcupine Worm (Hermione hystrix). The
tube-dwelling worms, which are closely allied to the preceding,
construct habitations of the most varied kind, which may either
(like the shells of Molluscs) be entirely secreted by their pos-
sessors, or may be made up of foreign bodies firmly cemented
together. We may contrast the long parchment-like tube of
HyaKncecia with the curved or undulating calcareous tubes of
Serpula (see p. 258) and its allies, and the spiral house of Spiror-
bis made up of similar material. In these forms, too, one of the
tentacles carried on the head is converted into a stopper or oper-
culum, by which the tube is closed when the animal is withdrawn.
The most varied foreign bodies are used for tube-construction,
nor are these necessarily associated in a haphazard manner. Of
this matter Benham (in The Cambridge Natural History] says: —
" But the process of tube-making is not a simple one, for in many
cases, at least, the worms exhibit definite powers of choice. Thus
some species of Sabella choose only the very finest particles of
mud; Terebella conchilega chooses fragments of shell and grains
of sand; Onuphis conchilega employes small stones more or less
of a size; Sabellaria makes use only of sand grains. Whilst
some worms, like Terebella, Nichomache, and others, make a very
irregular tube, Pectinaria builds a most remarkably neat house,
open at each end, which it carries about with it, the narrow end
uppermost; the grains of sand are nearly all of the same size and
only one layer in thickness, embedded in abundant ' mucus ', and
with the outer surface quite smooth."

Moss- Polypes (Poly zoo) are for the most part colonial animals,
collectively forming a mass of the most varied shape, flattened
(" sea-mats "), shrub-like, as encrustations on sea-weeds, &c. The
members of the colony are invested by horny coverings, some-
times hardened by calcareous deposits, and each of them can be
withdrawn into a little cup, which is in some cases closed by a lid
or operculum. The defences of the colony may be strengthened
by the addition of projecting spines.

Lamp- Shells (Brachiopoda) are enclosed in bivalve shells, but
these are quite different in character from those possessed by
bivalve molluscs, and are also differently placed in relation to the
body (see vol. i, p. 439). Each valve is lined by a fold of skin

340

ANIMAL DEFENCES

(mantle), and in nearly all cases the edge of this is provided with
projecting bristles, which probably serve as a protection, though
they may also constitute a sort of filter for sifting the inflowing
currents of sea-water (set up by ciliary action) by which the
animal gets its food and oxygen. The shells of some extinct
lamp-shells were beset with spines.

Echinoderms are remarkable for the calcareous plates which
are imbedded in the skin, and which are well seen, for instance,
in the ordinary Star- Fishes, Brittle- Stars, and especially in Sea-

Fig. 499. — A Sea-Urchin (Echinus lividits], showing
protective covering of spines

Fig. 500.— Part of Sea-
Urchin test, showing knobs
for attachment of spines.

Urchins, where they are united together in a very regular manner
by their edges, to form a " test ". Spines may also be present,
and these are particularly well -developed in the Sea- Urchins
(Eckinoids) (figs. 499, 500), which have earned their ordinary
name from this fact, for "urchin" is an old English word for
hedgehog. In some of these forms the spines (which are riot
fixed, but united to the test by ball-and-socket joints) are so long
and sharp as to constitute a defence of really formidable character.
Armoured Zoophytes (Ccelenterata). — Passive defence is not
the chief means of defence in this group, which, however, presents
many examples of it. The colonial branching Hydroid Zoophytes,
so often mistaken for sea-weeds, are invested in horny coverings

PASSIVE DEFENCE 34I

much as in the Moss- Polypes, but only some species are provided
with cups into which the individual members can be withdrawn,
nor are such cups provided with lids. The Organ-pipe Coral
( Tubipora musica) is another good instance. The colony is made
up of a number of calcareous tubes (composed of fused spicules),
one for each individual, and the members of the colony when
alarmed draw themselves back into their tubes. A common
British Sea- Anemone (Tealia crassicornis) gains protection by
means of small stones which adhere to its sticky body. When
the animal contracts, this stony covering not only forms a pro-
tection, but renders the creature very inconspicuous (see p. 289).

Armoured Sponges and Animalcules. — The bodies of most
Sponges are traversed in all directions by sharp needles of lime or
flint, some of which may project at the surface to form a chevaux-
de-frise. Anyone who has incautiously handled such sponges can
testify to the penetrating power of these structures (see vol. i.
p. 485). As most persons are only familiar with the ordinary
bath-sponges, it may not be superfluous to repeat what has
elsewhere been said, that this particular kind does not possess
spicules, its skeleton being merely a close network of fibres of
horny consistency.

Among the Animalcules {Protozoa) protecting shells charac-
terize the Foraminifera and Radiolaria (see vol. i, p. 489), and in
the former they may be made up of foreign particles cemented
together, though usually calcareous in nature. The shells of
Radiolarians are commonly flinty, and in many cases possess
radiating spines. Horny shells are present in some of the
freshwater Rhizopods allied to the Proteus Animalcule (Amoeba),
and particles of sand are worked into such shells in certain
species (e.g. Difflugia).

PASSIVE DEFENCE BY MEANS OF SPECIAL HABITS

There is no sharp boundary between this and defence by
armour, for it often happens that such armour is only fully effec-
tive when disposed in a particular manner.

A very interesting case in point is what may be termed the
ROLLING-UP HABIT, practised by animals of which only the upper
sides are properly defended. In the Three- Banded Armadillo or
Mataco ( Tolypeutes tricinctus), for example, this can be done very

342 ANIMAL DEFENCES

perfectly, the shape of the armour-covered head and tail enabling-
them to be snugly tucked away side by side. In the following
passage Darwin (in A Naturalist's Voyage) speaks of it thus: —
''It has the power of rolling itself into a perfect sphere, like one
kind of English wood-louse. In this state it is safe from the attack
of dogs ; for the dog, not being able to take the whole in its mouth,
tries to bite one side, and the ball slips away. The smooth, hard
covering of the mataco offers a better defence than the sharp
spines of the hedgehog." The scaly Pangolins of South Africa
and South- East Asia practise the same tactics. Both the Com-
mon Porcupine (Hystrix cristatus) and the Common Hedgehog
(Erinaceus Europczus] roll themselves up when threatened by
enemies, the quills or spines being at the same time erected.

Among Primitive Molluscs the curious Mail-Shells (Chiton,
&c.) are defenceless below, but are provided with eight protec-
tive plates above. These overlap in such a way as to form a
complete investment when the animal rolls up, as it is in the
habit of doing when alarmed. Two Arthropod examples will
serve to further illustrate the same method of defence. One is
that of the long since extinct Trilobites, rolled-up specimens of
which are frequently found in the fossil condition. Some members
of the group of land-crustaceans, popularly known as Wood- Lice,
have earned the name of " Pill Bugs " from the fact that they
practise this mode of defence, a common British species, the Pill
Wood- Louse (Armadillidium vulgare}, being a good illustration.

We now pass on to the consideration of habits not related to
the effective use of defensive armour.

DEATH -FEIGNING HABIT. — Many carnivorous animals which
pursue living prey will not condescend to touch dead bodies, and
hence the fact that a number of forms, when suddenly attacked or
hard pressed by their foes, pass into a state of " apparent death ",
may perhaps be explained as a defensive arrangement. Various
views have been held regarding this phenomenon. Some believe
it to be a deliberate action, an actual "feigning" of death, others
suppose it to be a kind of paralysis induced by fright, while the
suggestion has also been made that it is comparable to a state
of trance or catalepsy induced by hypnotic (mesmeric) influence.
It is not likely that the same explanation will apply in all cases,
and the heading of this section is adopted solely for convenience,
and does not express belief in the first way of explanation.

PASSIVE DEFENCE

343

Many stories have been related regarding the death-feigning
habit as exhibited by the Australian Dingo (Cam's dingo], and it is
so well exemplified by the Opossums of America, that " to play
'possum " has become a proverb. Hudson (in The Naturalist in
La Plata] gives a description of the " death-simulating swoon "
into which a species of South American Fox (Cants azarce] (fig.
501) falls if caught in a trap or worried by dogs, and expresses it

Fig. 501. — A species of South American Fox (Cants azarez)

as his opinion that the animal does not altogether lose conscious-
ness, though, judging from the fact that it bears without flinching
various cruelties practised upon it by gauchos, he also states:—
" I can only believe that the fox, though not insensible, as its
behaviour on being left to itself appears to prove, yet has its body
thrown by extreme terror into that benumbed condition which
simulates death, and during which it is unable to feel the tortures
practised upon it". Some Birds also feign death when hard
pressed, the Spotted Tinamou (Nothura maculosa) of the Pampas
being given as an example by the author just quoted. Lloyd
Morgan (in Habit and Instinct) quotes from Canon Atkinson
an amusing description of death-feigning by Land- Rails and
Water- Rails when caught. " A gentleman's dog catches a land-
rail and brings it to his master, unhurt, of course, as is the well-
trained dog's way, but to all appearance perfectly dead. The dog
lays the bird down at his master's feet, and he turns it over with

344 ANIMAL DEFENCES

his toe. It simply moves as it is moved, all its limbs limp. Con-
tinuing to regard it, however, the man sees an eye opened, and he
takes it up. The * artful dodger ' is quite dead again in a moment,
head hanging and dangling, limbs loose, and no sign of life any-
where. It is put in its captor's pocket, and, not liking the confine-
ment, begins to struggle. When taken out it is just as lifeless as
before; but being put down on the ground and left undisturbed — the
gentleman having stepped to one side, but continuing to watch —
it lifts its head in a minute or so, and, seeing all apparently serene,
it starts up on a sudden and * cuts its lucky ' with singular speed."

" In the case of the water- rail which came under my own
observation, it was picked up on a snowy day by the most intimate
of the friends of my youth and early manhood. He assumed that
it was dazed with cold, and perhaps what we Yorkshire folks call
'hungered' as well. So he brought it home with him, and laid
it on a footstool in front of the dining-room fire. Five minutes
passed — ten were gone — and still the lifeless bird lay as it was
put down, dead to all seeming; only not stiff, as it ought to have
been if dead of cold as well as hunger. A few minutes later, my
friend, who was very still, but yet with an eye to the bird, saw
it — not lift its head, like the land-rail, and take a view, but — start
off in a moment with no previous intimation of its purpose, and
begin to career about the room with incredible rapidity. It never
attempted to fly. Any other captive bird in its position would
have made for the window at once, and beaten itself half to pieces
against the glass. Not so the rail. With it, in its helter-skelter
and most erratic course, it was anywhere rather than the' window
or the fire. Round the room, across the room, under the sofa,
under the table, from corner to corner, steering itself perfectly,
notwithstanding legs of chairs, legs of tables, the sofa-feet, foot-
stools, or what not, on and on it careered ; and it was not without
some patience and many attempts that it was eventually secured."

There are also some Reptiles which feign death when attacked,
and a Lizard which illustrates this habit is described by Darwin
(in A Naturalist's Voyage), when speaking of the fauna of Bahia
Blanca on the Argentine coast: — " Of Lizards there were many
kinds, but only one (Proctotretus multimaculatus] remarkable from
its habits. It lives on the bare sand near the sea-coast, and from
its mottled colour, the brownish scales being speckled with white,
yellowish red, and dirty blue, can hardly be distinguished from

PASSIVE DEFENCE 345

the surrounding surface. When frightened, it attempts to avoid
discovery by feigning death, with outstretched legs, depressed
body, and closed eyes. If further molested, it buries itself with
great quickness in the loose sand. This Lizard, from its flattened
body and short legs, cannot run quickly." Similar observations
have been made as regards some Amphibians.

Spiders are notorious for the way in which they simulate death,
drawing in their legs and remaining perfectly motionless until an
opportunity of escape offers. The same habit is characteristic of
many Beetles.

FECUNDITY OF ILL-DEFENDED ANIMALS

Many comparatively defenceless animals are extraordinary fer-
tile, and this may be looked upon as a special means of defence for
the species (which might otherwise become extinct), though it is
of no service to the individual. A great many ill-defended forms,
indeed, would seem to have as their chief function the furnishing
of a food-supply to other animals. The astonishing fertility which
compensates for such ravages has enabled many such weak crea-
tures not only to hold their own, but actually to become the most
abundant species on the face of the earth. The Rodents, among
Mammals, furnish a good example of this. They are the most
cosmopolitan of their class (with the possible exception of bats),
and are represented by a very large number of species and an
enormous number of individuals. Yet these creatures are mostly
of small size, and are exposed to the attacks of innumerable
enemies against which they are not particularly well defended.
On the average, it may be said that the total number of individuals
of any particular rodent remain fairly steady, i.e. the ravages of
enemies prevents the rapid production from having any marked
effect; but how largely such animals help to feed more powerful
forms has been demonstrated by cases where, for some special
reason, the toll levied upon them has been lessened. The Rabbit,
for example, is proverbial for its rapid powers of increase, not-
withstanding which its numbers in this country appear to remain
much about the same from year to year. But when this same
animal was introduced into Australia, a country comparatively free
from predaceous forms, it multiplied with such enormous rapidity
as to become a serious nuisance to mankind. In Europe all this

346 ANIMAL DEFENCES

superfluous rabbit-flesh would have been used up as food by more
powerful animals.

Insects again, as we have had occasion to learn, furnish food
to a great variety of other animals, notwithstanding the large
number of protective devices exhibited within the limits of their
class. And it is only by means of immense powers of increase
that the ravages of their enemies are made good; indeed so great
are these powers, that there are more insects in the world than
any other land animals. A single illustration will perhaps suffice
(taken from Wallace's Darwinism). Speaking of the rapid in-
crease of organisms this author says: — " In the lower orders this-
increase is particularly rapid, a single flesh-fly (Musca carnaria)
producing 20,000 larvae, and these growing so quickly that they
reach their full size in five days ; hence the great Swedish
naturalist, Linnaeus, asserted that a dead horse would be devoured
by three of these flies as quickly as by a lion. Each of these
larvae remains in the pupa state about five or six days, so that each
parent fly may be increased ten thousand-fold in a fortnight.
Supposing they went on increasing at this rate during only three
months of summer, there would result one hundred millions of
millions of millions for each fly at the commencement of the
summer — -a number greater, probably, than exists at any one
time in the whole world. And this is only one species, while
there are thousands of other species increasing also at an enor-
mous rate ; so that, if they were unchecked, the whole atmosphere
would be dense with flies, and all animal food and much of animal
life would be destroyed by them." The following example will
serve to show how the numbers of a particular species may be
kept down by even a single kind of enemy.

" During a very cold winter in the district of Hanau several
thousand old oaks were cut down, in the hollow trunks of which
many tens of thousands of bats sheltered during the rigorous part
of the year. As the trees were felled and sawn into pieces most
of these useful animals perished, either of cold or maltreatment at
the hands of boys. In the following year much larger numbers
than usual were seen of the Oak Procession- Moth (Cnethocampa
processioned), and for a number of years after the caterpillars of
this moth became a most destructive pest in the Hanau district,
over an area of miles in circumference. Not only were oaks
stripped of leaves, but also a large number of other forest trees

PASSIVE DEFENCE 347

as well as fruit trees. Before this time procession caterpillars were
not wanting from the Hanau district, but the large number of
bats flying about in the night had caught and devoured so many
moths that great increase of this pest was prevented. When
almost all the bats in the district had been destroyed, a great
caterpillar infestation could not be avoided, the insect being freed
from almost all its enemies. For the mature winged stage is of
nocturnal habit, and exposed only to the attacks of bats and
goatsuckers, the latter never being present in more than limited
numbers. The caterpillars are so well protected with hairs that
scarcely anything but cuckoos can devour them, and the chrysa-
lides are sheltered by a thick cocoon from the attacks of most
enemies. The eggs alone are largely eaten during the winter by
gipsy migrants (tits, tree-creepers, nuthatches, &c.). The pro-
cession-moth, in fact, has always so few enemies, that it invariably
increases largely in numbers if the chief of them happens to be
rapidly exterminated." (Extracted from Gloger, by Ritzema Bos,
in Animal Friends and jFoes.

CHAPTER XXIX
ANIMAL DEFENCES— ACTIVE DEFENCE

PASSIVE DEFENCE having now been pretty fully considered,
we may pass on to ACTIVE DEFENCE, remembering at the same
time that this may be preceded or followed by retreat.

ACTIVE DEFENCE AGAINST ENEMIES

For this purpose either ordinary AGGRESSIVE WEAPONS may
be used, or else those special structures to which the name of
ACTIVELY DEFENSIVE WEAPONS is applicable, and these may be
parts of the body which are commonly employed for other pur-
poses.

AGGRESSIVE WEAPONS IN DEFENCE

The possibility here suggested is so very obvious that there
is no need to say very much about it. A carnivorous Mammal,
e.g. a Leopard, depends upon teeth and claws for capture and
immolation of its prey, but it is clear that these same weapons
are just as valuable for purposes of self-defence, if the occasion
should arise. This it is pretty sure to do, for aggressive forms
have numerous enemies, just as well as the more peaceful vege-
tarians. The beak and talons of a bird of prey, the formidable
teeth of a crocodile, the poison-fangs of a venomous snake, the
sting of a scorpion, the strong jaws of a tiger-beetle, and the
stinging-cells of a jelly-fish will serve as further instances of
aggressive weapons which may be of equal use for defensive
purposes.

ACTIVELY DEFENSIVE WEAPONS

Actively Defensive Weapons of Mammals. — Apes and
Monkeys, especially the larger species, defend themselves very
effectively with their teeth, the canines of the male often being

348

ACTIVE DEFENCE

349

in the form of large tusks. The large man-like apes, when driven
to bay, prove formidable antagonists to man himself, and once
grasped by their powerful limbs the sequel is apt to be disagree-
able. Wallace (in The Malay Archipelago] thus describes the
result of an attack by some of the Dyaks of Borneo upon a Mias
or Orang-utan: — "A few miles down the river there is a Dyak
house, and the inhabitants saw a large Orang feeding on the
young shoots of a palm by the river-side. On being alarmed he
retreated towards the jungle which was close by, and a number
of the men, armed with spears and choppers, ran out to intercept
him. The man who was in front tried to run his spear through
the animal's body, but the Mias seized it in his hands, and in an
instant got hold of the man's arm, which he seized in his mouth,
making his teeth meet in the flesh above the elbow, which he
tore and lacerated in a dreadful manner. Had not the others
been close behind, the man would have been more seriously
injured, if not killed, as he was quite powerless; but they soon
destroyed the creature with their spears and choppers. The man
remained ill for a long time, and never fully recovered the use
of his arm." The Chimpanzee and Gorilla of tropical Africa
would appear to be equally formidable when driven to defend
themselves, though many exaggerations are current, largely based,
no doubt, upon the accounts given by natives.

Some of the social monkeys combine for the purpose of
defence, and may even resort to the use of missiles, an often-
described case being that of certain Baboons, which, when hard
pressed among the rocks, hurl down stones among the intruders.

The formidable tusks which are constituted by the upper
incisors of the Walrus form no mean weapons of defence, but this
is apparently their least important use. These animals mainly
employ them in digging up the bivalve molluscs, &c., which serve
as their food, and also to assist in progression on ice or land.
They figure besides in those furious fights between the males
which form a regular episode in the season of courtship, and this
particular use for weapons is one of which many examples can
be given, especially noteworthy being cases where, as, e.g., in most
deer, the male only is provided with structures suitable for such
a purpose. More will be said about this elsewhere.

Elephants are so powerful that they are pretty free from the
attacks of other animals, man only excepted, nor do they court

35o ANIMAL DEFENCES

attack. When obliged to fight, however, their tusks are pretty
nearly as useful as those of the walrus, being, however, of different
nature, i.e. they are not canines but huge incisors, and continue
to grow through life. The powerful trunk is also used in defence,
and many a tiger has found to his cost that to be stepped or
knelt upon by an elephant is a serious matter.

The different species of Rhinoceros are as well if not better
defended than the elephant, but in an entirely different manner.
The chief weapons of the African forms are the two sharp horns,
which are entirely epidermal in nature. Of these the front and
longer one is carried on the nasal region, and the other one farther
back. The common Indian Rhinoceros has only one horn, cor-
responding to the first of these, though it is by no means so
formidable as a weapon, this being fully compensated, however,
by the presence of two sharp tusks in the lower jaw, which are
used like the tusks of a wild boar. Many accounts have been
given of the ferocity of rhinoceroses, but the balance of evidence
appears to show that they are, for the most part, only dangerous
when hard pressed by enemies and thoroughly aroused. The
different species, however, and different individuals of the same
species, would appear to differ greatly in this matter.

Horses and their allies fall next to be considered, and in these
the first instinct appears to be retreat, though when forced to
defend themselves they are able not infrequently to discomfit their
enemies by vigorous kicks, the formidable nature of which is
greatly enhanced by the hard hoofs, structures whose primary use
is related to swift locomotion. The powerful teeth are also em-
ployed against some of their enemies. The following quotation
from Vogt (Natural History of Mammals) regarding wild horses,
illustrates combination for defence on the part of social animals: —
" The herds live under the leadership of some old males, which
have to watch over the well-being of their subjects. We cannot
but admire the courage of these proud creatures, which, seeming
to rejoice in battle, dart down upon an attacking carnivore, the
whole herd arranging itself in a circle with the foals in the middle,
and all ready to strike with the hoofs of their hind-legs. In
fighting with wolves, stallions try to seize their antagonists with
their teeth by the nape of the neck, then to lift them up and dash
them to the ground, after which they trample them underneath
their feet. But these battles, from which perhaps the military

THE FALLOW DEER (DAMA VULGARIS)

DRAWN FROM THE LIFE BY F. SPECHT

ACTIVE DEFENCE 351

art has derived the formation of squares, are only exceptions to
the rule, and take place only in cases of sudden attack or when
the herds are driven to straits. Usually the herd seeks its safety
in rapid flight. Tearing along in furious gallop, with ears and
mane erect, the herd dashes away with the speed of the wind,
driving their young ones before them, the males galloping on the
flanks and at the end of the column to protect the herd in its
hurried flight." The military square, however, does not seem to
have been copied from the horse. It is believed to have been
invented by Sir William Wallace, who arranged his pikemen in
serried circles to resist the onslaughts of the English knights at
the battle of Falkirk. The squaring of these circles, if the ex-
pression may be permitted, was a further stage in evolution.

Man is the only serious enemy of the Hippopotamus, which
is an animal of peaceable disposition except when attacked, but is
then very formidable on account of its immense strength, backed
by an armoury of tusk-like incisors and canines. Most Swine
rely for defence on their upwardly -directed tusk -like canines,
which are kept sharp by constantly rubbing against one another,
and are particularly well developed in the male. These creatures
are naturally peaceable, but when brought to bay, thrusts from
their sharp tusks, given laterally or else from below, are capable
of ripping up most antagonists. The little Peccaries of South
America defend themselves by biting, as their canines, though
sharp, are small, and not adapted for thrusting, the upper ones
also being turned downwards, as is usually the case among
Mammals. They live in large herds, and co-operate for defence
against enemies, man included.

Most Ruminants are provided with defensive weapons in the
form either of antlers or horns. Antlers, characteristic of deer,
are bony outgrowths usually possessed by the male only, and
shed annually. Although the two last facts are enough to prove
that the primary object of these structures is not defence, their
hardness and sharpness nevertheless makes them very effective for
this purpose. Sir Samuel Baker (in Wild Beasts and their Ways]
describes as follows the way in which on one occasion, in Ceylon,
an Axis or Spotted Deer (Cervus axis] defended himself: — " I saw
Killbuck reach the flank, but before he had time to make a spring,
the stag threw his head upon one side, and backwards, so as to
strike the dog with the extreme points of his long antlers. A

352 ANIMAL DEFENCES

short time after, the stag came to bay upon firm open ground,
and fought the dog face to face. I saw Killbuck rush straight at
the deer's face, and instead of receiving the attack passively, the
deer quickly lowered his head, and not only met, but charged, the
dog, rolling him over, and following him up as he drove his sharp
tines deep into his body."

The horns of Antelopes (fig. 502), Oxen, Sheep, and Goats, are
quite different from antlers, and have earned for these creatures
the name of "hollow-horned" Ruminants (Cavicornid). Many
female antelopes do not possess them, but in some species of that
group, and in the other animals mentioned, they are present in
both sexes, though always more powerful in the male. The
hornless or " polled" condition of certain races of cattle, sheep,
and goats is a result of domestication. The horns of these
various creature are epidermal structures, and consist of un-
branched hollow sheaths composed of the material indicated by
their name, supported by " horn cores ", which are conical bony
outgrowths from the skull. Unlike antlers they are never shed.
Goats and Sheep defend themselves by butting with their horns;
Oxen, Buffaloes, &c., and at any rate some of the Antelopes, use
them for stabbing, or goring, to employ the word more specially
applicable.

Horns commonly extend so far to the side, as in buffaloes, or to
the back, as in many antelopes, that they can only be used for side-
thrusts, which, however, may be very effective. Both in African
and Indian Buffaloes (in which latter the horns sometimes exceed
12 feet from tip to tip) part of the defensive tactics consists in
trying to force the enemy to the ground, either by " charging " or
utossing " him. Should this move be successful, a combination of
goring and trampling generally polishes off the assailant.

The Sable Antelope {Hippotragus niger) is one of the most
powerful of his kind, the backwardly-curved horns being often
quite 3 */2 feet long in the male, though somewhat shorter in the
female. When attacked, this animal is said to lie down, apparently
inviting attack from behind. By lateral movements of the head
the horns can then be swept over the back, transfixing any foe
that has been rash enough to attack that part of the body. Selous
says: — "The sable antelope is often very savage when wounded,
and, like the roan antelope and gemsbok, will commit terrible
havoc among a pack of dogs. Indeed, I have known one to kill

ACTIVE DEFENCE

353

Fig. 502. — Horns of Gazelles

i, GazelLi Saemmeringt, male; 2, G. dorcas, male; 3, G. dorcas, female; 4, G. Cjcvieri, male; 5, G. Cuvieri,

female; 6, G. Arabica, male; 7, G. Granti, male; 8, G. euchore, male.
VOL. II. 55

354 ANIMAL DEFENCES

three dogs with three successive sweeps of its long scimitar-shaped
horns." Even the lion sometimes meets his match in this well-
defended form.

Kangaroos seek safety in flight, but when forced to defend
themselves, can use their powerful hind-legs with considerable
effect. Semon (in In the Australian Busk] states that this
animal, when " driven to bay, will seek its last refuge by leaning
its back against a tree and defending itself against its pursuers by
kicking and scratching with its hind-legs, the fourth toe of which
bears a long and pointed claw. On carelessly approaching an old
kangaroo male bent on his defence, dogs will often be clutched by
his fore-legs, suffocated by his powerful embrace, or scratched to
death. . . . Some kangaroos, when at their wits' end, sometimes
manage to escape into a river or lagoon, in the deep water of
which they stand fully erect and drown any dog swimming up to
them." This description, of course, applies to the artificial con-
ditions of a kangaroo hunt conducted by mounted men with dogs.
But it is clear that the powers of swift locomotion and active
defence possessed by these animals were evolved in relation to
older conditions, and were amply sufficient to enable them to hold
their own against natural enemies, of which, as usual, man was by
far the most formidable.

The Skunk (Mephitis sujfocans} has already been mentioned as
an example of warning-coloration among Mammals (see p. 301).
If the warning be neglected and an attack made, the animal
ejects a superlatively offensive fluid from its stink-glands, and in
most cases escapes with its life.

Actively Defensive Weapons of Birds and Reptiles. — The
larger running birds, such as Ostrich and Emeu, are able to
defend themselves very effectively by kicking, it being said that
the former is not far inferior to a horse in this respect. Although
not provided with special defensive weapons, many birds of social
habit, such as rooks, combine for the purpose of repelling the attacks
of enemies, more particularly when these are birds of prey.

Among Reptiles, it is possible that the two American Lizards
(species of Helodermd] (fig. 503), which are the only two members
of their order known to be poisonous, use their special weapons
rather for defence than offence. These consist of slender grooved
teeth loosely attached to the jaw, and resembling the fangs of
venomous snakes. At the base of each tooth is a small poison-

ACTIVE DEFENCE

355

gland, the secretion of which is fatal to small mammals, and suffi-
ciently virulent to be dangerous even to human beings.

Actively Defensive Weapons of Amphibia and Fishes. — Re-
garding Amphibians, it need only be stated here that the poison-
glands in the skin, already spoken about (see p. 304), are in many

Fig. 503. — Poisonous Mexican Lizard (Heloderma horridum}

cases sufficiently potent to deserve mention under active defences
as well as under passive.

Fishes are not infrequently provided with poison-spines (fig. 504)
of varying degrees of complexity in structure, but all serving the
purpose of defence. The most remarkable examples are found
among some of the ordinary bony forms (Teleostei\ The most
elaborate case is that presented by certain small ground-fishes
(species of Thalassophryne] from the coasts of Central America.
These creatures possess four sharp spines, two on the back
and one on each gill-cover or operculum. Each of these spines
is constructed on the same plan as the poison-fang of a viper,
being traversed by a canal which is open at the base and also
on one side near the tip, an arrangement which prevents blockage
when in use, just as in the needle of a hypodermic syringe. A

356

ANIMAL DEFENCES

small poison-bag is situated at the base of each spine, and should
some other animal blunder against the sharp point of this, the
pressure causes the venom to be ejected into the wound. An
equally if not more effective arrangement is possessed by some
fishes (species of Synanceia) from the tropical parts of the Indian

Fig. 504. — Poison Spines of Fishes

A, Spines on tail of an Eagle-Ray (Aetobatis) (part of one drawn on larger scale at A'). B, Dorsal spine of Synanceia-
p.gl. poison-bags; c, Side view of gill-cover in Thalassophryne : p.sp projecting tip of spine; p.gl. poison-bag
(exposed by dissection), c', Spine isolated, the projecting tip on right and bristle placed in canal.

and Pacific oceans. Here there is a series of sharp dorsal spines,
of which the extremities are grooved at the sides, a small poison-
bag lying in each groove. Of these fishes Glinther (in The Study
of Fishes) says: — " The native fishermen, well acquainted with the
dangerous nature of these fishes, carefully avoid landing them ; but
it often happens that persons wading with naked feet in the sea,
step upon the fish, which generally lies hidden in the sand. One
or more of the erected spines penetrate the skin, and the poison

ACTIVE DEFENCE 357

is injected into the wound by the pressure of the foot on the
poison-bags. Death has not rarely been the result." Two of our
native sea-fishes, the Greater and Lesser Weevers (Trachinus
draco and T. viperd] are poisonous to a less extent, being pro-
vided with grooved spines on the back and operculum. There
are here no poison-bags, but the slime which covers the spines
has venomous properties.

Among Elasmobranchs, the Sting- Rays (Trygonidce) and Eagle-
Rays (Myliobatida) commonly possess one or more saw-edged
spines on the tail, by the lashing movements of which they can
be used to inflict jagged wounds, these injuries being at the same
time rendered more dangerous by the poisonous nature of the slime
which covers the spines. The range of one species, the Common
S ting- Ray (Trygon pastinacd), includes the south coast of England.

Actively Defensive Weapons of Mollusc a. — In the tropical
Cone- Shells the horny ribbon of the rasping organ bears barbed
teeth, upon each of which a poison-gland opens (p. 97). The
living animal requires careful handling, for it promptly makes use
of its means of defence. Another arrangement is found in some of
those Sea-Slugs (species of sEolis) in which a number of club-
shaped outgrowths (ceratd) spring from the back, for these struc-
tures are armed with stinging-cells much like those characteristic
of jelly-fishes, sea-anemones, &c.

In some of the large Bivalve Molluscs the mere closing of the
shell constitutes a defence which may prove fatal to an attacking
animal. A good example of this is given by Semon (in In the
Australian Busk), where, in speaking of the natural history of
Torres Straits, he says: — "A certain precaution has to be observed
when collecting on the reefs. In the shallow water lie, their sides
unfolded, the gigantic Tridacna-shells, such as are sometimes used
in Europe as a font for holy water in Catholic churches, or as
ornaments in halls and gardens. Woe to him who, in wading
through the water, carelessly touches one of them. Many a
searcher of tripang has met with this accident, and has had his
foot cut through to the bone by the shells, which shut up with
enormous force. No human power can open the shell, and a
man thus caught can only be relieved by his companions cutting
the adductor muscles of the shell with a knife."

Actively Defensive Weapons of Insects. — The most formidable
structure calling for description here is the sting with which the

353

ANIMAL DEFENCES

tip of the abdomen is provided in bees, wasps, and ants. This
weapon may be used in some cases more for offence than defence,
as in those digging wasps (see p. 106) which lay up a store of
spiders or insects for the benefit of their larvse, but in ordinary
bees its chief use would appear to be that of defence. The hard
parts of a Bee's sting (fig. 505) consist of three rods, of which one

acts as a " director", along which the
other two can be moved backwards
and forwards, each of them presenting
a longitudinal groove which works
along a corresponding ridge. Each of
these two " piercers " is a kind of
flattened stylet, the tip of which is
studded with a number of barbs.
There are two poison-glands secret-
ing respectively an acid and an alka-
line secretion, and opening into a blad-
der-like sac. This in its turn pours
its fluid into a sort of reservoir formed
within a swelling at the base of the

director, and thence it is conducted to

Fig. 505. -sting of Bee the wound along a channel between

on the left one of the piercers is shown in th Director and the two piercers, each

side view, much enlarged. On the right a cross

section through the sting, very highly magni- Q£ the latter POSSCSSinPf a OrOieCtlOn

fied: a a, the director, on the upper side of * ~

which are two ridges, £ b, along which the two which acts as a piston. It is well

piercers slide. , 111 •

known that when a bee uses its sting,

the piercers, being barbed at their tips, cannot be withdrawn, and
are left sticking in the wound, while the attempt to withdraw them
commonly proves fatal to their possessor. The sacrifice of the
individual, however, benefits the species, for insectivorous animals
commonly avoid bees (see p. 307). Female Wasps and Ants
possess weapons of similar character, but the piercing stylets are
not barbed, and can therefore easily be withdrawn, so that the
individual is not liable to perish for the benefit of its kind.

Insects not uncommonly possess variously-situated glands that
secrete an offensive, or it may be acid, fluid, by the sudden ejec-
tion of which enemies may be discomfited. A well-known instance
is that of the Bombardier- Beetle (J3rackinus crepitans], which is
provided with glands connected with the last part of the intestine
(rectum). These secrete a volatile fluid which can be suddenly

ACTIVE DEFENCE

359

ejected with a slight noise, giving at the same time the appear-
ance of a minute puff of smoke. One of the little Click- Beetles
(Lacon murinus), of which the larva attacks corn-crop, is provided
with a pair of stink-glands, which open near the tip of the abdomen
and secrete an extremely-offensive fluid. The protection afforded
would appear to be considerable, for it is stated on good authority
that when attacked this beetle makes no attempt to escape, but is
content with assailing the nose of the enemy with evil smells. In
the Mole-Cricket {Gryllotalpa campestris) there are stink-glands
in much the same position as in the Bombardier- Beetle, and glands
of similar kind open on the upper side of the abdomen in Earwigs.
The order of Bugs (Hemiptera) has been much neglected, even by
specialists, and this is no doubt partly due to the fact that very many
of them, especially the plant-feeding forms, are provided with stink-
glands, of which the secretion is decidedly offensive. It is a
curious fact that in many such species the young are provided
with glands of the kind which open on the upper side of the
abdomen, but these are replaced in the adult by similarly-endowed
structures opening on the sides of the thorax. The larval glands
would not be of much use if retained, for their openings would be
covered over by the wings.

The Stick- Insects (Phasmidce) are provided with defensive
glands in the thorax, of which the secretion is reputed to be
extremely acrid as well as malodorous. It can be ejected with
considerable force, and it is stated that blindness may result
should it happen to get into the eye (compare p. 303). The
caterpillar of the Puss Moth (Cerura vinula] is one of the forms
which assume a terrifying attitude (see p. 313), and to this mode
of defence is added the one now under consideration. Lodged in
the front part of the body there is a gland secreting an irritant
fluid containing as much as 40 per cent of formic acid, and this
can be squirted out upon an attacking enemy, proving most
effective when it happens to hit the eye. Poulton (in The
Coloiirs of Animals] speaks as follows of this secretion and its
properties: — " So far as we know at present, no other animal
secretes a fluid containing anything which approaches this per-
centage [40] of strong acid. . . . The value of this strongly-
irritant liquid is sufficiently obvious. I have seen a marmoset
and a lizard affected by it, and have myself twice experienced
sharp pain as the result of receiving a very small quantity in the

36o ANIMAL DEFENCES

eye. Although the secretion is therefore useful as a defence
against vertebrate enemies, it is probably chiefly directed against
ichneumons." As regards the latter point, it is known that this
larva is very liable to the attacks of a particular species of
ichneumon-fly (Paniscus cephalotes), which lays her eggs upon
its skin. The larvae which hatch out from these use the unfor-
tunate caterpillar as a food-supply. Experiment has shown that
the acid secretion is either immediately fatal or else highly in-
jurious to ichneumons upon which it happens to fall.

The hairs which clothe many caterpillars produce a highly
irritating effect upon the mouths of insectivorous animals (or
the fingers of human beings), and this is partly the result of their
being barbed, besides which they are probably endowed with
poisonous properties, like the spines of certain fishes (see p. 355).
Such aggressive hairiness is associated with warning-coloration
(see p. 301). A typical example is that of the Palmer Worm,
which is the larva of the Gold-tail Moth (Porthesia auriflua),
and is rendered conspicuous by its markings of white spots and
red lines upon a black ground. The barbed hairs are not the
only disagreeable point about this caterpillar, for it also possesses
defence-glands opening on the upper surface of the body, and
secreting an acrid fluid.

Actively Defensive Weapons of Myriapods and Peripatus. —
Centipedes would appear to be sufficiently defended by the
possession of poison-jaws, primarily weapons of offence. Milli-
pedes, howeve^ are non-aggressive vegetarian creatures devoid of
such structures, and repel the attacks of their enemies by means
of numerous stink-glands, which open on the sides of the body and
secrete an offensively-smelling fluid containing prussic acid.

The slime-glands of Peripatus, which open on two papillae
near the mouth, would appear to be special means of defence,
though, in the New Zealand species at any rate, they are also
employed in the capture of prey. Speaking of the Cape species
Sedgwick says (in The Cambridge Natural History]'. — " They will
turn their heads to any part of the body which is being irritated
and violently discharge their slime at the offending object ".

Actively -Defensive Weapons of Lower Invertebrates. — The
numerous and often powerful setae with which many of the marine
Bristle-Worms are provided would seem, in some instances at
least, to serve for active as well as passive defence, though there

ACTIVE DEFENCE

361

Is an absence of observations on the subject. At any rate, many
of them are extremely sharp-pointed, and often serrated in such
a way as to render their possessor a somewhat unpleasant morsel
of food.

Many of the Planarian Worms, when irritated, discharge large
numbers of microscopic rods (rhabdites] from the skin, and it is
likely that these possess irritant properties. Some few members
of the same group are also provided with stinging- or nettling-cells,
much like those of sea-anemones, jelly-fish,
and similar creatures, and these are un-
doubtedly defensive structures.

Among Echinoderms we find that some of
the Sea -Urchins are provided with poison-
spines (fig. 506), reminiscent of what has been
described for certain fishes (see p. 355). Such
a spine has a swollen end in which a poison-
bag is lodged, this communicating with an
•excessively-sharp perforated style, by which
the wound is inflicted and poison introduced
into it.

The innumerable nettling- or stinging-cells
with which the members of the great phylum
Ccelenterata (jelly-fishes, sea-anemones, corals,
<&c.) are provided undoubtedly serve as actively-
defensive weapons, though perhaps it is right
to primarily regard them as means whereby
active prey is paralysed and secured (see
p. 158). The brightly-coloured Sea- Anemones
are richly endowed with these protective struc-
tures, and it is a matter of observation that most fishes leave
them severely alone. Probably the vivid hues which they flaunt
are to be looked upon as examples of " warning-coloration", and
the same thing is very likely true for Corals. But, as already
remarked, every means of defence is more or less met by counter
devices among aggressive forms, and to some of the coral-reef
fishes the stinging -cells have no terrors. The Parrot -Fish
(Scarus), for instance, browses upon corals, the hard parts of which
are effectively tackled by its firm parrot-like jaws, while the sting-
ing-cells are not able to injure the hard lining of its mouth.

The Slipper-Animalcule (Paramcecium] is a good example of

Fig. 506. — Poison-spine of a
Sea-Urchin (Asthenosoma urens)
in longitudinal section: p,p, holes
in side of the hollow spine; tn,
layers of muscle. Diagrammatic
and enlarg

362 ANIMAL DEFENCES

a defensive arrangement found among a number of the higher
Protozoa. The outer layer of this creature's body is packed with
innumerable rod-like bodies (trichocysts)^ which can be shot out
when their owner is irritated, much like the rods in the skin of
some Planarian Worms, and the rods would appear to possess
irritant properties. It must be remarked, however, that after
these structures have been brought into play their possessor dies,
breaking up into a number of pieces; so that, as in the case of
the Honey-Bee (see p. 358), the arrangement is one by which
the species benefits at the expense of the individual.

CO-OPERATION FOR ACTIVE DEFENCE AMONG SOCIAL ANIMALS.
—Some of the herbivorous Mammals which live in communities
co-operate for defence in a very skilful manner, though, to begin
with, retreat is usually the first proceeding, and it is only when
obliged to make a stand that they turn upon their enemies. A
good instance, that of Wild Horses, has already been quoted
(see p. 350), and space prevents further examples from being
given in this section.

CHAPTER XXX
ANIMAL DEFENCES— RETREAT

Having now considered Active and Passive Defence at con-
siderable length, it only remains to speak of that method which
has been described as a "strategic movement to the rear", in
order to make the section complete.

Animals which are subject to the attacks of powerful enemies
often possess very considerable powers of locomotion, and in such
cases retreat is usually the first instinct acted upon, though it not
infrequently happens that such creatures, when actually driven to
bay, are able to give a very good account of themselves. It is re-
cognized that in human warfare a successful retreat is the most
difficult of all tasks to perform, though in this case both sides are
fully provided with aggressive weapons, and innumerable devices
are adopted to baffle the pursuing enemy. Ingenious methods
answering the same end are by no means unknown among animals,
and several of them will be mentioned in their proper place.

Retreat among Mammals. — As might be expected, the more
intelligent Monkeys display a good deal of method in the way they
effect a retreat, some of those which live in troops placing sentinels
before they begin their plunder of a native crop, and when alarmed
retiring in good order, taking every advantage of cover.

Vogt (in Mammalia] gives the following lively account of
the mode of life among Baboons (fig. 507), which shows how
carefully arrangements are made by such creatures for retreat
when alarmed: — " So far as our information goes, it would seem
that all baboons live mostly in considerable troops, often number-
ing several hundred, and in these there are always several old
males and females, so that the leadership does not, as among
most other monkeys, fall to a single patriarch. . . . The troop
passes the night in caves in the rock, and in grottoes in inac-
cessible precipices, all closely huddled together, and at sunrise

363

364

ANIMAL DEFENCES

Fig. 507. — Baboons retreating from Wild Dogs

RETREAT 365

they slowly and deliberately quit their retreat in search of food.
Large stones are often overturned by their united efforts in order
to seek for any animals that may have crawled under them, such
animals forming, along with roots, tubers, juicy leaves, and fruits,
their chief nourishment. After that the company bask in the sun
with their backs turned to the wind, the older ones sitting on stones,
while the young tumble and play about. The old, meanwhile, keep
a careful watch all round; the troop next go to some water to
drink, and after supper they betake themselves once more to rest.
For the most part a troop sticks to the same feeding-ground, for
some time at least, but from time to time it changes its ground.

" On the approach of any danger warning sounds are heard,
and the females and the young then crowd together, while the
old males, like the champions of the ancient Greeks, advance into
the foremost of the fight uttering fearful cries, bellowing, and
gnashing their teeth. A bold and proud spirit with contempt of
death is beyond question a characteristic trait of the baboons,
and when Brehm records a case in which an old Arabian male
baboon gradually managed to extricate a young one, which had
been left behind on a rock surrounded by dogs, from the midst
of its assailants and before the very eyes of the hunters, inspiring
by its determined bearing both dogs and hunters with such respect
for its powers that no attack was ventured on, we may well agree
with Darwin in saying that here was a proof of heroism of which
only few men were capable."

The characteristically herbivorous order of Ungulates illus-
trates a number of points in connection with retreat from enjmies.
It includes, for instance, many examples of extraordinary fleetness,
as in the case of horses and their allies, deer, antelopes, and
giraffes. It is usual for such forms to live in social communities,
the safety of which while feeding is provided for by the posting
of sentinels. A well-known instance of the latter habit is afforded
by the Alpine Chamois (Rupicapra tragus\ which feeds during
the day in small herds of about a score individuals, the welfare of
which is watched over by an old female, who takes up a position
giving a wide outlook, and warns her associates of danger by a
sharp whistling cry. Retreat is often cleverly effected by such
gregarious forms, as, for instance, by the little antelopes known
as Duyker-Boks (species of Cephalophus], which "duck" down
among the bushes among which they feed in a way which has

366 ANIMAL DEFENCES

earned for them their Boer name. They also run in a remarkable
zigzagging manner. The Common Koodoo (Strepsiceros kudu\
a large and handsome antelope, is described as rushing in retreat
through thickets of " wait-a-bit ", and other thorny plants, in a
way that must prove effective in baffling most pursuers, besides
which old bulls of this species are credited with rendering them-
selves inconspicuous by lying down against a suitable bush and
raising their heads so that the large twisted horns rest on the
back, a position in which they are not likely to attract attention.

Perhaps the most interesting device which helps to save many
of the weaker Ungulates from their inveterate enemies, the larger
Carnivora, is the habit of " ruminating " or " chewing the cud ",
which is characteristic of one large group, the Ruminantia. The
structural arrangements related to this have been described
elsewhere (p. 168), and its protective nature is obvious. A
ruminant animal can crop a large quantity of food very rapidly,
swallowing it without proper mastication, and then retreating to
some comparatively safe place, where the " cud " can be chewed at
leisure. In this process the food again returns to the mouth in
successive boluses, and is swallowed for a second time after being
properly divided and saturated with saliva.

Many of the Mammalia possess Dwellings, to which they
retreat when attacked. Rabbits furnish a particularly good ex-
ample, and everyone who has attempted to shoot these animals
is familiar with their habit of popping into their burrows with
lightning-like rapidity on the least alarm. A favourite time for
feeding is just when it begins to get dark, and it is then that the
neutral colour of the fur harmonizes best with the general sur-
roundings so as to render detection a matter of difficulty. Like
so many other social forms rabbits appear to make arrangements
for the safety of the community by the more experienced members
giving warning of approaching danger. This purpose appears
to be served, for example, by the curious and emphatic way in
which the old bucks stamp on the ground when alarmed. It is
somewhat remarkable that an animal like the rabbit, which is
protectively coloured, should have a white under-surface to the
tail, being thereby made particularly conspicuous when in motion.
This has been interpreted with some probability as a case of
''signalling coloration", enabling the rapid retreat from danger
of an individual to be quickly seen by other rabbits in the neigh-

THE CHAMOIS (RUPICAPRA TRAGUS)

DRAWN FROM THE LIFE BY F. SPECHT

RETREAT 367

bourhood. Such a warning is, of course, given unconsciously and
is quite on a different footing from the stamping above mentioned,
which one must presume is meant by the giver to indicate the
approach of danger to his fellows.

Another burrowing rodent which lives in communities is the
Prairie- Dog (Cynomys Ludovicianus) of the western prairies of
North America. It is asserted by several observers that in this
case sentinels are regularly set for the common good.

Burrowing animals surprised by enemies when at some dis-
tance from their homes commonly attempt, with more or less
success, to take refuge in the ground. Of this a notable example
is the Small Armadillo (Dasypus minutus] of South America,
which, when overtaken by a horseman, is said, if the soil is
favourable, to be able to burrow a safe distance into the ground
before its pursuer has time to dismount, unless he literally falls
off his horse.

Arboreal mammals, if surprised on the ground, naturally take
to the trees if they have the chance, and they may do this in a
very cunning way. If, for instance, an ordinary Squirrel (Sciurus
vulgaris) (fig. 508), surprised in the act of feeding upon fallen
cones or beech-mast, be hotly pursued, it will naturally make for
the nearest tree, which it will ascend with great rapidity, always
taking the greatest care to direct its flight so that the trunk or
branches intervene between itself and the aggressor. It has
also been suggested that the squirrel's brush is not only a means
of balancing, but also helps escape from carnivorous enemies
which, attacking from behind, may at the last moment have to
be content with a mouthful of fur. Possibly also, though this is
mere conjecture, the delicate and easily -detached tail of the
Dormouse (Muscardinus avellanarius] may serve as a kind of
sop to the foe, on the principle which has before now led to the
escape of sledge-travellers from pursuing wolves, which have
checked their pace in order to investigate articles of clothing and
other objects thrown out for their inspection.

Aquatic mammals in retreat of course take to the water if
they get a chance, and some terrestrial forms, such as Deer and
Kangaroos, select the water in order to make a last stand against
dogs, seeming to realize that attack is thus made more difficult.

Birds in Retreat. — Among the large Running Birds are forms,
like the African Ostrich, in which the absence of powers of flight

368

ANIMAL DEFENCES

is largely compensated by the specialization of the legs for the
purpose of rapid movement on the ground. For straightforward
retreat in open country nothing could be more effective; but
another kind of adaptation is required in birds like Rails, which,
though belonging to the Flying Birds, are deficient in powers.

Fig. 508. — The Common Squirrel (Sciurus vidgaris)

of flight, and yet are able to run through thickly-growing vege-
tation with such rapidity as to commonly elude their enemies.
This is rendered possible by the shape of their bodies, which are
relatively narrow and flattened from side to side, so as to easily
slip between the stems of grasses, rushes, and similar plants.
Anyone who has pursued our native Land- Rail or Corn- Crake
(Crex pratensis] with intent to capture will have noted how
extremely difficult it is even to get within sight of a bird of this
sort. Certain birds, unfortunately for themselves, have lost the

RETREAT

369

power of flight without correspondingly increased powers of
running, and have paid the penalty of extinction. Such an
arrangement, as might be anticipated, was the result of evolution
in islands devoid of any predatory ground-animals, and a classic
example of it is afforded by the Dodo and its allies, birds related

Fig. 509. — The Dodo (Didus ineptus

to the pigeons. The Dodo itself (Didus ineptus) (fig. 509) was
a large and clumsy-looking species that at one time abounded
in the island of Mauritius, which, like oceanic islands generally,
possessed no native mammals, while its indigenous reptiles were
only represented by lizards. The ubiquitous sailor, however, and
the animals (especially swine) which he introduced, brought about
the extinction of this helpless bird in less than a century after
its first discovery in 1598. Its memory is now only kept green
by a few contemporary drawings and descriptions, certain museum

VOL. II.

37o ANIMAL DEFENCES

remains, and the proverb " as extinct as a dodo ". As Belloc
(in The Bad Child's Book of Beasts] pathetically sings :—

"The Dodo used to walk around,

And take the sun and air,
The Sun yet warms his native ground —

The Dodo is not there!
That voice that used to squawk and squeak

Is now for ever dumb —
Yet may you see his bones and beak

All in the Mu-se-um."

A similar fate must overtake any organism suddenly exposed
to new and unfavourable conditions, if devoid of sufficient plas-
ticity to rapidly accommodate itself to the altered environment.

Ordinary flying birds naturally betake themselves to flight
when attacked, but this is often insufficient to save them from
predatory species of their own class. Under such circumstances
the flight may be conducted in such a way as to increase their
chance of escape. Houssay (in The Industries of Animals]
gives a good instance of this, drawing the facts from Naumann
(Naturgeschichte der Vogel Deutschlands]'. — " Larks, a feeble
race of birds, rise higher in the air than any rapacious bird, and
this is often a cause of safety. Their greatest enemy is the
Hobby (Hypotriorchis sub lutes]. They fear him greatly, so that
as soon as one appears singing ceases, and each suddenly closes
his wings, falls to the earth, and hides against the soil. But some
have mounted so high to pour out their clear song that they
cannot hope to reach the earth before being seized. Then,
knowing that the bird of prey is to be feared when he occupies
a more elevated position from which he can throw himself on
them, they endeavour to remain always above him. They mount
higher and higher. The enemy seeks to pass them, but they
mount still, until at last the Hobby, heavier, and little accustomed
to this rarefied air, grows tired and gives up the pursuit."

Climbing Birds, such as Woodpeckers, pursue the same kind
of tactics as squirrels (see p. 367), working round a trunk or
branch so as to keep it between them and the enemy. The
powers of diving possessed by many aquatic birds naturally stand
them in good stead when retreating from foes.

Reptiles in Retreat. — The great majority of reptiles are timid
and seek refuge in flight on the first alarm. Some Lizards, when

RETREAT 37I

hard pressed, are able to baffle their foes in a somewhat singular
manner, comparable to some extent to the supposed way in which
squirrels may escape pursuers with no more serious loss than a
mouthful of fur from the bushy tail. Here, however, there is
no doubt about the facts. A Lizard's tail very readily breaks
off, owing to the existence of a weak place in the backbone,
specially evolved, we must imagine, to facilitate such an arrange-
ment. While the enemy is negotiating the piece of tail the animal
itself commonly manages to make its escape. Such a sacrifice
is not so great as might at first sight appear, for a new tail soon
grows from the stump. In our native snake -like lizard, the
Blindworm (Anguis fragilis\ which has earned its specific name
from the possession of such a brittle tail, a further point was
noted by the late J. G. Wood. According to him, the detached
tail executes lively movements for some time, a performance
which would be likely to divert attention during the escape of
its owner. In some of the Australian Geckos the detachable
tail is broadened out at the end into a coloured flap, which is
supposed to fix the attention of an enemy on a part which can
readily be surrendered without fatal result.

The cylindrical smooth body of a Snake or Snake-like Lizard
is well suited for slipping through a dense undergrowth of
vegetation, and is a great advantage in retreat.

Amphibia in Retreat. — One case only of special interest
will be mentioned. Hudson (in The Naturalist in La Plata]
describes a " Wrestler Frog ", possessing extremely muscular
fore-limbs, and which, when followed up, makes a feint attack
upon its enemy before retreating. Part of his account may well
be quoted here: — "The frog is a most timid, inoffensive creature,
saving itself, when pursued, by a series of saltatory feats un-
paralleled amongst vertebrates. Consequently, when I find a
frog, I have no hesitation in placing my hands upon it, and the
cold sensation it gives one is the worst result I fear. It came
to pass, however, that I once encountered a frog that was not
like other frogs, for it possessed an instinct and weapons of
defence which greatly astonished me. I was out snipe-shooting
one day when, peering into an old disused burrow, I perceived
a burly-looking frog sitting within it. It was larger and stouter-
looking than our common Rana, though like it in colour, and I
at once dropped on to my knees and set about its capture.

372 ANIMAL DEFENCES

Though it watched me attentively, the frog remained perfectly
motionless, and this greatly surprised me. When I was sufficiently
near to make a grab, it sprang straight at my hand, and, catching
two of my fingers round with its fore-legs, administered a hug
so sudden and violent as to cause an acute sensation of pain;
then, at the very instant I experienced this feeling, it released its
hold and bounded out and away." The specimen, however, was
captured, but later on managed to escape from the box in which
it was imprisoned, and Mr. Hudson failed to secure another
individual of the same sort. " That this singular frog has it in
its power to seriously injure an opponent is, of course, out of
the question; but its unexpected attack must be of great advan-
tage. The effect of the sudden opening of an umbrella in the
face of an angry bull gives, I think, only a faint idea of the
astonishment and confusion it must cause an adversary by its
leap, quick as lightning, and the violent hug it administers; and
in the confusion it finds time to escape. I cannot for a moment
believe that an instinct so admirable, correlated as it is with the
structure of the fore-legs, can be merely an individual variation;
and I confidently expect that all I have said about my lost frog
will some day be confirmed by others. Rana luctator [i.e. the
Wrestling Frog] would be a good name for this species."

Retreat among various Invertebrates. — Among Mollusca we
find a very interesting and well-known device for securing a safe
retirement when attacked by foes in Cuttle-Fishes and the like.
These animals possess an ink-bag (the secretion of which was the
original source of the pigment " sepia") from which a dark fluid can
be ejected at will. The result is the production of a cloudiness in
the water for a considerable distance, under cover of which the
Cuttle- Fish commonly manages to beat a successful retreat. Hick-
son (in A Naturalist in Celebes] thus describes this proceeding: —
". . . I often saw advancing slowly over the sea-gardens, in parties
of from four to six, a group of cuttle-fish, swimming with an even
backward movement, the fringes of their mantles and of their arms
trembling, and their colour gradually changing to what seemed to
me to be an almost infinite variety of hues as they passed over the
various beds of the sea-bottom. Then suddenly there would be
a commotion in what was previously a calm and placid scene, the
striped and speckled reef-fishes would be seen darting away in all
directions, and of the cuttle-fishes all that remained were four or

RETREAT 373

five clouds of ink in the clear water. The appearance in the
neighbourhood of a small shark or other kind of voracious fish was
the cause of this sudden agitation, and the cuttle-fishes, after
squirting out a cloud of ink to * throw dust in the eyes ' of their
enemy, had, by violent contractions of their mantle, made off."

Certain kinds of Land -Snails have been described which often
succeed in escaping their enemies by the same kind of device as
that described for Lizards. This is the case, for instance, with a
number of small forms of the kind (species of Helicarion) inhabit-
ing the Philippines. These crawl rapidly upon trees by means of a
long narrow foot, of which the hinder part is very conspicuous and
bears a projecting horn-like process. If such a snail, when crawl-
ing, is seized from behind, this tail-like part of the foot is jerked
convulsively up and down till it becomes detached, when its owner
promptly drops to the ground or conceals itself among adjacent
leaves.

A common British Land-Snail ( Vitrina pellucida) uses the
tail-like hinder part of the foot as a springing organ, by which it
can throw itself to the ground if alarmed when crawling along
in an exposed situation. Some Sea- Snails possess jumping powers
of no mean order (e.g. species of Strombus and its allies) which
must often be of use in helping them to retreat from enemies.
Even some of the bivalve molluscs are endowed with similar
powers, as is notably the case in the Common Cockle (Cardium
edule], and here again rapid and sudden retreat from foes is no
doubt one of the ends served.

Some of the Social Insects, especially Ants, wage warfare very
scientifically, and their retreats are carried out in good order, regu-
lated by tactical principles. There are also insects with special
powers of fighting in retreat An example is the Bombardier
Beetle (Brachinus crepitans], in which the hinder part of the intes-
tine is provided with glands which secrete a volatile fluid possessed
of noxious properties. When one of these creatures is pursued by
a large carnivorous beetle it ejects small portions of the fluid,
which immediately vaporizes, producing what looks like a tiny puff
of smoke. At the same time a sharp report is heard, resulting
from the sudden ejection, and the arrangement is one calculated
to discomfit an enemy.

Spiders combine death-feigning with retreat, falling from their
webs when attacked as if the victims of sudden dissolution. They

374

ANIMAL DEFENCES

do not forget, in so falling, to spin a thread of silk by which to climb
back to their home in case the manoeuvre is successful. Some
Caterpillars also, if alarmed when feeding on a branch, will sud-
denly let themselves drop and remain suspended by a thread of

silk, and so also do some Slugs (fig.
510), though in their case the thread
is of different character.

Many of the Crustacea are able
to retreat in an effective manner, and
this is obviously possible with power-
ful swimmers such as Lobsters and
Prawns, which, when pursued, make
at full speed for some sheltering hole
or crevice. The Common Shrimp
(Crangon vulgaris\ and similar forms,
swim off quickly when alarmed, cloud-
ing the water as they do so by scuffling
up the sand on which they live. Before
the water clears itself again they will
be found to have neatly buried them-
selves in the sand, though even then
they are not safe from fishes which
use the sense of smell to guide them
in the chase, as many do. Some of
the little crabs commonly seen on our
shores are pretty nimble, but their
pace is contemptible compared with
that of the Swift Sand-Crabs (Ocypo-

didce) of African and American shores, which scuttle away at a
great rate when frightened, and are also able to bury themselves
in the sand with extreme rapidity. Crustacea are further distin-
guished by the readiness with which they part with their limbs,
and many of them have doubtless survived frequent hairbreadth
escapes by pursuing this policy, reminding one of the case of
Lizards already noted (see p. 371). Members lost in this or any
other way quickly sprout again.

The power of regeneration alluded to above is present to a
much greater extent in segmented worms or Annelids. A marine,
freshwater, or terrestrial worm of this kind, if overtaken by an
enemy, is nevertheless not at the end of its resources, for even

Fig. 510. — Slug suspended by a thread of
hardened slime

RETREAT 375

a large piece of its body, if bitten off, can be renewed. And cases
must frequently occur where the front end of such a creature
manages to make its escape while the pursuer is devouring the
piece which he has captured. It has been plausibly suggested
that segmentation of the body, in its first origin, was evolved
as a defensive measure. A lower form constructed in this way
consists of a series of rings or segments, sometimes extremely
numerous, each of which contains a portion of most of the organs,
digestive, excretory, nervous, &c., so that even the loss of a con-
siderable number of segments leaves the animal sufficiently well
provided in these respects to carry on its existence till the wound
is healed and the parts destroyed have grown once more. In the
higher segmented forms (Arthropods, Vertebrates) the body has
become so specialized that this mode of defence has been mostly
or entirely given up, though even in so highly developed an
animal as a Lizard the part which can be relinquished and re-
newed, i.e. the tail, is made up of the posterior segments of the
body.

ANIMAL RESPIRATION— THE BREATH

OF LIFE

CHAPTER XXXI

ANIMAL RESPIRATION— GENERAL PRINCIPLES-
BREATHERS IN WATER AND BREATHERS IN AIR

GENERAL PRINCIPLES

The essential nature and purpose of breathing or respiration
have been explained elsewhere (vol. i, p. 45), but it may be de-
sirable in this section to call attention to a few points of general
interest. The living substance of which the bodies of animals are
more or less composed is of exceedingly unstable nature, and is
constantly breaking down into simpler substances (see p. 3), this
process of waste being continually counterbalanced by the taking
in of food, which is built up into fresh body-substance. An animal
may, in fact, be regarded as a self- repairing machine. In the last
section we have considered very fully the food and feeding of
animals, and have seen that the necessity for repair of waste, and
for growth to a certain size, exert a very far-reaching influence
upon bodily characteristics and habits.

We are now more particularly concerned with the other part of
the cycle of chemical changes which incessantly goes on within
the living body, i.e. with the down-breaking processes or processes
of waste. It is these which yield the obvious or actual energy
necessary for the performance of all the vital actions, including
the heat which is so characteristic of warm-blooded active creatures
like mammals and birds. Such down-grade chemical processes, as
they may perhaps be called, are quite comparable to those which
go on in a burning candle or lamp, i.e. they are a kind of combus-
tion. And both in the case of the animal body, and in the case of

376

GENERAL PRINCIPLES 377

lamps or candles, combustion cannot go on without continual
access of fresh supplies of air, for the sake of the oxygen gas
which is contained in it. The burning of a candle is due to a
process of what is known chemically as oxidation, the oxygen of the
air uniting with the wax of the candle, which as a result is trans-
formed into simpler chemical substances, chiefly water (H2O) and
carbonic acid gas (CO2). If a cold dry tumbler is held over a
burning candle a sort of mist condenses upon the inner side of the
glass, and this is some of the water in question. And again, if a
candle-end is burnt for a time in a tumbler into which a small
quantity of clear lime-water has been poured, a little gentle shak-
ing will be followed by a milky appearance in the lime-water, due
to the formation of carbonic acid gas as one result of the burning.
This gas unites with the lime-water so as to produce minute par-
ticles of carbonate of lime, which give rise to the milkiness. The
slow combustion constantly going on in the animal body similarly
causes its complex living substance to break down into a number
of much simpler compounds, among which are to be found both
water and carbonic acid gas. Such compounds, being of no use,
are called waste products \ and have to be passed out of the body.
And one of the reasons why an animal is obliged to breathe is that
it may get rid of carbonic acid gas, together with a large amount
of water. You can easily satisfy yourself that this is true as re-
gards a human being by directing some of your outgoing breath
against a cold looking-glass, when a film of moisture (i.e. water)
will be seen. And the presence of carbonic acid gas in such
breath can be proved by breathing into some clear lime-water,
which will at once become milky.

Breathing or respiration has a double purpose, for it not only
gets rid of waste products, but is also the means by which the
oxygen necessary for promoting the breaking-down processes of
the body is taken into the system. This oxygen is absorbed into
the blood, or, in some of the simpler animals, what corresponds to
it, and is taken to all parts of the body, in order that they may
" waste " and give up their share of the energy necessary for
working the different organs.

Some of the smallest and simplest animals breathe by the
general surface of the body, but in higher forms, especially those
which live on land, this is not possible, and all sorts of compli-
cated breathing arrangements exist. We get, in fact, special

378 ANIMAL RESPIRATION

breathing or respiratory organs, the special duty of which is to
take in oxygen and get rid of carbonic acid gas and water. Nor
must we underestimate the importance of this kind of work, since
it may be stated broadly that the activity and intelligence of an
animal are proportionate to the efficiency of its breathing organs.
For rapid movement means quick wasting of the muscles, and this
is not possible unless abundant oxygen is supplied and waste
products speedily removed. And great intelligence is associated
with rapid oxidation of brain-substance, and similar quick re-
moval of waste substance. It may also be said that, as a general
rule, efficient breathing organs are associated with efficient cir-
culatory organs. The most active and most intelligent groups of
animals now existing are undoubtedly mammals, birds, and insects,
all of which are particularly well off in the matter of breathing'
arrangements.

One very important principle in regard to such organs must be
carefully borne in mind. Great efficiency as regards breathing
organs means the existence of a large surface in close proximity to
the blood, so that on the one hand oxygen may pass in, and on
the other carbonic acid gas and water pass out. And we shall
find that a large surface may, by various devices, be packed away
in a comparatively small space, and further, that it is often asso-
ciated with complicated arrangements by which constant and rapid
renewal of air is provided for.

BREATHERS IN WATER AND BREATHERS

IN AIR

All animals, without exception, are breathers or intakers of air,
or rather of the oxygen gas which it contains, and it is incorrect to
speak, as is sometimes done, of " air-breathers " as contrasted with
" water-breathers ". Some animals, such as whales, though living
in water, have from time to time to come to the surface to obtain
air for breathing, but a host of aquatic creatures, such as most
fishes, crustaceans, marine molluscs, &c., do not find it necessary to
do this. The oxygen which they need is not, however, obtained
from the water (H2O) itself, though this consists of oxygen united
or combined with hydrogen. But such animals do not possess the
power of decomposing water for the sake of the oxygen which
enters into its composition. If they did, free hydrogen gas would

BREATHERS IN WATER AND BREATHERS IN AIR 379

constantly be given off from the sea, which we know is not the
case. The requisite oxygen is, in fact, dissolved in the water,
which has absorbed it from the atmosphere. Anyone who has
tried to keep animals in an aquarium knows that a shallow vessel
is better for the purpose than a very deep one, and this is because
there is a relatively large surface by which oxygen can be absorbed.
In a very large and deep aquarium either the water must be con-
stantly renewed, or air must be frequently pumped in.

Relation between Plants and the Breathing of Animals. — The
statement made in the last sentence requires a certain amount of
qualification, for not only do animals depend in the long run upon
plants, as regards food, but there is an intimate connection be-
tween them in the matter of breathing. It ought to be clearly
understood that plants breathe exactly in the same way as animals,
so far as essentials are concerned, taking in oxygen gas and giving
out carbonic acid gas. Yet, as regards ordinary green plants, the
statement is often made that " animals breathe in oxygen and
breathe out carbonic acid gas, while plants breathe in carbonic
acid gas and breathe out oxygen ". This is absolutely incorrect,
and why it is so will become apparent if we consider for a moment
how, and upon what, green plants feed. Such plants act, so to
speak, as intermediaries between the mineral and animal kingdoms,
for they convert very simple substances into the materials of
which their own bodies are composed, and thus prepare food for
animals, as these depend upon a diet of very complex chemical
nature (see p. 270). The food of a green plant consists, in fact,
of water with mineral substances dissolved in it, and also of car-
bonic acid gas, this too being dissolved in water in the case of
aquatic plants. It is the last kind of food with which we are here
concerned. The green colouring matter (chlorophyll) which is
characteristic of ordinary plants enables the living substance (pro-
toplasm) with which it is associated to use sunlight in such a way
as to bring about chemical action between water and carbonic acid
gas. The result of this is twofold, for in the first place a sub-
stance is formed which is a step upward in complexity, and in the
second place oxygen gas is liberated as a sort of by-product. It is
this oxygen gas which is given out by the plant and erroneously
supposed to be a result of breathing, whereas it really has to do
with the feeding. It is so large in amount as to greatly exceed,
during the daytime, the carbonic acid gas which is breathed out by

380 ANIMAL RESPIRATION

the plant. We may therefore say that animals, by constantly
breathing out carbonic acid gas, keep up the supply of a substance
green plants require as a part of their food. Green plants, on the
other hand, in the course of their feeding, constantly give out large
quantities of oxygen gas, and keep up the supply of that element
necessary for breathing purposes. And even if animals could dis-
pense with green plants as food, the absence of such plants would
probably soon result in such a diminution of the oxygen in the air
and such an increase in its carbonic acid gas, that ordinary animal
life would become impossible. There is thus a constant, though,
of course, quite unconscious, exchange of good offices between
animals and green plants, and the composition of the air is kept
uniform for indefinitely long periods of time. It is, however, quite
possible, as some have maintained, that in very remote geological
periods the air contained a much larger percentage of carbonic
acid gas than it does now, and this was possibly one of the causes
that led to the luxuriant growth of plants during the geological
period (Carboniferous) from the deposits of which most of our
coal is obtained.

It is now clear why the remarks already made (see p. 379) about
the difficulty which aquatic animals have in breathing, when kept in
a deep aquarium, need qualification. For if there is an abundant
plant-growth in such an aquarium, a large amount of oxygen is
given off which can be breathed by the animals present. When
bright sunlight is allowed to fall upon the plants in such an aqua-
rium, small bubbles of gas may often be seen to collect on them,
and this, when tested, proves to be oxygen.

In the succeeding chapters of this section it will be convenient
first of all to consider the typical breathers in water, since they
represent what must have been the original state of things.
Typical breathers in air will next be discussed, and afterwards
forms which are in process of transition from breathing in water
to breathing in air, while mention will also have to be made of
air-breathing forms which have reverted to breathing in water for
part, at least, of their existence.

CHAPTER XXXII

ANIMAL RESPIRATION— VERTEBRATES THAT BREATHE

IN WATER

MID BRAIN

EYE

'TWIXT BRAIN
PORE BRAIN

CEREBRAL
HEMISPHERE

HIND BRAIN

One of the essential characters of Vertebrate or Backboned
animals is the possession, during part or all of life, of slits in the
side of the throat (visceral clefts) by which that part of the diges-
tive tube which immediately succeeds the mouth-cavity (i.e.
pharynx) communicates
with the exterior (fig.
511). These slits pri-
marily have to do with
breathing, and for that
reason may be called
gill-clefts, since in many
cases gills y which are
organs specially con- VISCERAL ARCHES

cerned with aquatic
breathing, are found
as outgrowths of their
sides. By studying the
development of a fish
it is possible to follow
the stages in the for-
mation of gill -clefts.

The sides of the pharynx grow out into a series of pouches which
first of all come into contact with the skin and then fuse with it.
Later on, by the absorption of tissue, the actual openings or clefts
come into existence.

When, however, we enquire how gill-clefts came to be devel-
oped in the remote and so far unknown ancestors of vertebrates,
it is impossible to do more than speculate in a general sort of way
upon the matter. We know, as a matter of fact, that almost any

381

HEART

Fig. 511. — Front part of Chick Embryo. Enlarged

382 ANIMAL RESPIRATION

part of an aquatic animal to which water has access may help in
breathing if its covering or lining is sufficiently thin for the oxygen
dissolved in the water to diffuse in, and the carbonic acid of the
blood to diffuse out. The lining of the digestive tube is commonly
sufficiently delicate to satisfy this condition, and since an aquatic
animal takes in more or less water with its solid food, it therefore
follows that in many marine worms, &c., breathing is to some
extent performed by the tube in question. It is also obvious that
the beginning of the digestive canal is most favourably situated
for the purpose, and it is therefore not surprising to find that
aquatic breathing has in many instances been localized in the
pharynx. A breathing organ, to be efficient, must possess a suffi-
ciently large surface for exchange of gases between the blood and
the surrounding medium; hence any folds or pouches in the sides
of the pharynx would add to its utility in this particular direction.
And here the very pertinent question naturally presents itself—
" Is there any reason for thinking that the lowly and long-extinct
creatures from which Vertebrates have sprung possessed a pouched
digestive tube?" This question may be, with some probability,
answered in the affirmative, for Vertebrates are segmented animals
(see p. 375), i.e. are divided from before backwards into a series
of parts or segments which, in such a low type as the Lancelet
(Amphioxus), resemble one another pretty closely. The original
purpose of segmentation appears to have been lost in Vertebrates
(see p. 375), and it is to be looked upon as an inherited character
which they have had to make the best of. If we examine some
of the segmented Invertebrates, especially the segmented worms
(Annelids), we shall find that the digestive tube often bulges out
laterally in each segment, and this is the very feature which might
lead to the formation of gill-pouches in the region of the pharynx.
How and why these pouches first came to open externally is a
very difficult problem, but we know that such structures may com-
municate with the exterior, for in some of the Sea- Slugs (Nudi-
branchs, see p. 357) there are tubular branches of the digestive
tube which open upon horn-like projections (cerata), probably for
the purpose of getting rid of certain waste products. A similar
reason may explain the origin of gill-slits in the remote past. Be
that as it may, such openings once established would greatly
increase the efficiency of the pharynx as a breathing organ, for
they would afford a means of exit for water taken in at the mouth,

VERTEBRATES THAT BREATHE IN WATER 383

and such water would then be regularly taken in for respiratory
purposes instead of passing in casually with the food. Though,
judging from analogy with some Crustacea and other animals where
the last part of the intestine is pressed into the service of breath-
ing, there may have been a stage before gill-slits were evolved,
in which breathing-water was taken into the pharynx at regular
intervals and as regularly ejected from the mouth after doing its
work. Leaving the realm of speculation, we come to the breathing
arrangements found among the chief groups of Fishes, and among
the simply-organized forms known as Protochordates.

FISHES AS BREATHERS IN WATER
LAMPREYS AND HAGS (CYCLOSTOMES)

One of the most interesting forms as regards breathing organs
In this very ancient and primitive group is the Californian Hag-
Fish (Bdellostoma) of the Pacific coast. This is an elongated
eel -shaped creature with suctorial mouth, devoid of anything

Fig. 512

A, Californian Hag-Fish (Bdellostoma], reduced, showing external apertures of the gill-pouches. NA, Unpaired
nostril. B, Diagrammatic cross-section of same, showing two gill-pouches connected with the gullet and the
exterior. Course of the breathing-water indicated by arrows.

comparable to a lower jaw. On each side of the body are to be
seen, a little distance behind the head, some seven to fifteen
small round holes, one behind the other (fig. 512). These are
the external openings of a corresponding number of gill-pouches.
Dissection shows that the pouches communicate internally with
the digestive tube by similar apertures, and that the lining of
each pouch is raised into a large number of thin folds, arranged
almost like the leaves of a book, and presenting a very large
surface for purification of blood. The heart contains nothing
but impure blood, loaded with carbonic acid gas and deficient
in oxygen. This it pumps to the gill-pouches, where dissolved
oxygen is taken up from the surrounding water, which receives

3^4

ANIMAL RESPIRATION

in exchange, so to speak, the waste carbonic acid gas. The
purified blood is then distributed to all parts of the body (fig.

5i3).

In cases where organs of any sort are repeated to form a
series, it frequently happens that increased efficiency is gained

DORSAL AORTA

POST. CAROL.

Fig. 513. — Diagram of Circulatory System in a Fish

The direction of blood-flow is indicated by arrows; vessels containing pure blood left unshaded, those containing;
impure blood are darkly shaded. The capillaries of head, gills (GGGGG), digestive tube (DIG.), liver (LR.), trunk,
and fins are represented as net-works.

The heart essentially consists of an auricle (AU.) and ventricle (VENT.), and pumps impure blood to the gills, where
it is purified, afterwards passing to the dorsal aorta for general distribution. The impure blood is returned to the
heart by paired anterior cardinal (ANT. CARDL.) and posterior cardinal (POST. CAROL.) veins. The liver receives pure
blood by a liver-artery (LR. ART.) and also impure blood (from stomach, intestines, pancreas, and spleen) by an hepatic
portal vein (H.P.V.). Its impure blood is returned to the heart by a liver-vein (LR.VN.). The complex circulatory
arrangements of the liver are known as the 'hepatic portal system". (DIG. ART.), Artery carrying pure blood to
stomach, &c. The gills and gill-vessels of one side only are indicated.

by a reduction in number, usually accompanied by specialization
of the surviving members of the series. Good examples are
furnished by the reduction in number of the teeth of some Car-
nivores (see p. 7), of walking-legs in Insects as compared with

COURSE OF FOOD __v ,GULUET

Fig. 514. — Diagram showing the gill-pouches (G) of one side in a Lamprey (Petromyzon)

Scorpions or Centipedes, and of digits in the Horse, which has
one large efficient toe as contrasted with the five of a Man. And
when we examine the breathing organs of a Lamprey (Petro-
myzon) we find only seven pairs of gill-pouches, a great reduction
as compared with the Californian Hag- Fish. Nor do these
pouches open directly out of the digestive tube, but they com-
municate internally with a breathing tube which underlies and
is connected in front with this (fig. 514).

VERTEBRATES THAT BREATHE IN WATER

385

V.AO.

A very interesting and curious specialization is found in the
Common Hag- Fish (Myxine), a form which abounds on the east
coast of Scotland. For here there are only
two external openings, situated rather far
back on the under side of the body. On
dissection we find that there are six pairs
of gill-pouches, opening directly out of the
digestive tube, but those of each side com-
municate externally with a canal which
opens to the exterior by one of the two
openings already noted (fig. 515). This is
probably connected with the habit this fish
has of attacking cod, &c., in a moribund
condition, and eating its way into their
bodies. This practice would interfere with
breathing if the gill openings were arranged
as in a Lamprey. A further interesting
point about the Hag is the existence of a
useless seventh gill-passage behind the
pouches of the left side, and this vestige
clearly indicates that the fish is descended
from forms possessing more numerous
gill-pouches than it does itself. Such
a conclusion is borne out by the facts A> auricle >v> ventricle; V.AO. ventral

» aorta taking impure blood to the gill-

already mentioned regarding the number pouches; PA. pharynx; a and G6,

. ' 1 . . , f. | first and last gill-pouches of left side;

of gill -pouches in other members ot the G ?, vestige of seventh gm-pouch on

left side ; X X, the two external gill
grOUp. apertures

Fig SIS. _ circuiatory Breathing

SHARKS AND RAYS (ELASMOBRANCHS)

A description has already been given (vol. i, p. 257) of the
structure of the Spotted Dog- Fish (Scy Ilium canicula), which is
in effect a small shark. In this creature, as in most of its kind,
five gill-clefts are readily seen on each side of the body a little
way behind the head, and these are the external openings of a
corresponding number of gill-pouches which communicate inter-
nally with the pharynx (fig. 516). Projecting into each pouch are
a large number of folds, collectively presenting a large surface for
purification of the blood. The breathing-pouches of a Lamprey
are supported and prevented from collapsing by a basket-work of

VOL. II. 57

386

ANIMAL RESPIRATION

horny fibres, and there is a similar arrangement in the Dog- Fish,
for the thick partitions (gill-arches) between the gill-pouches are
supported by jointed hoops of gristle, which unite with one another
below, and answer the same purpose. Special muscles are attached

to this framework, which act so as to
adjust and promote the outward flow of
water through the gill-clefts.

There is good reason for thinking
that the number of gill-clefts in a Dog-
Fish or Shark is a reduction upon an
earlier state of things, and it is interest-
ing to note that two existing kinds of
Shark (Hexanchus and Heptanchus] pos-
sess respectively six and seven pairs of
such clefts ; besides which, it has recently
been discovered that in one species of
Dog- Fish there are remains (vestiges)
of a sixth pair of clefts behind the last
existing pair. It is also to be observed
that reduction has taken place not only
from behind, but also from in front, for
just behind the eye of a Dog-Fish or
Shark there is a small round hole, the spiracle, with which the
pharynx communicates by means of a narrow passage (spiracular
cleft). That this is an old gill -cleft which is being utilized for
other purposes is conclusively proved by the fact that it contains
some small folds which are undoubtedly the last traces of a dis-
appearing gill. These folds are known as the false-gill (pseudo-
branck). The new use to which this old gill -cleft is being put
is the transmission of sound to the essential organs of hearing,
which are enclosed in a gristly ear -capsule adjoining the front
wall of the cleft. In a Skate (Raia batis] there is a thin place
in the wall of this capsule to facilitate the conduction of sound.
This is a particularly striking case of "change of function", and
it acquires much greater interest when we come to investigate
the structure of the organs of hearing in such forms as mammals,
birds, reptiles, and amphibians. For outside the ear-capsule of
these animals there is a cavity or " drum ", covered externally by
a tense membrane, from which sound-waves are carried across
the drum by one or more little bones (see vol. i, p. 56) to the

Fig. 516. — Horizontal section through the
Breathing Organs of a Dog-Fish or Shark,
showing the gill-pouches. The gills are
deeply shaded and the course of the breath-
ing water indicated by arrows, t, Tongue;
gl, gullet.

VERTEBRATES THAT BREATHE IN WATER 387

essential organs of hearing contained in the capsule. This drum
is the equivalent of the spiracular cleft of a Dog- Fish, and it is
therefore clear that the backboned animals which live on land
have pressed an old gill-cleft into the service of their hearing
organs, which is in itself sufficient proof that these animals have
descended from aquatic gill-possessing ancestors.

In some fishes of the Shark kind the internal openings of the
gill-pouches are guarded by slender projections ("gill-rakers"),
the use of which is to prevent foreign bodies or bits of food from
passing into the pouches and choking them up.

SEA-CATS (HoLOCEPHALi) ,

The members of this small group are closely related to the
Sharks, but are in advance of them so far as breathing organs are
concerned. The best-known of them is the Sea-Cat, or King of
the Herrings (Chinuzra monstrosa). There is here no spiracle,

Fig. 517.— The Sea-Cat (Chimara monstrosa], male

and the gill-clefts, which are close together, are reduced to four
in number. Externally they are protected by a flap or gill-cover
(operculum), supported by gristle (fig. 517). We shall see in the
sequel that various arrangements have been evolved in different
sorts of aquatic animals for the protection of the gills, for these
structures are necessarily of delicate texture (or exchange of
gases could not take place through their walls), and are not
only liable to be injured by grains of sand and the like, but are
also much favoured by parasites, which find in them a sheltered
home provided with a very rich supply of blood.

388

ANIMAL RESPIRATION

ORDINARY BONY FISHES (TELEOSTEI)

In such fishes as Salmon, Perch, or Cod there is further
specialization in the breathing organs, resulting, it would seem,
in increased efficiency, which is no doubt one reason why this
youngest group of fishes is predominant at the present day.

There is no spiracle, and the gill-slits are
usually Jive in number (though they may be
fewer), and close together. The most remark-
able peculiarity, however, concerns the gill-folds,
and it is one to which Chimaera and Sturgeons
lead up. Each such fold, instead of being
attached along its whole length to the side of
a deep gill-pouch, is here present as a thread
or filament, fixed by one end to a comparatively
narrow gill-bar (fig. 518). The gill-bars are,
as it were, the greatly-reduced representatives
of the partitions which separate the gill-pouches
in a Dog- Fish or Shark. An ordinary bony
fish possesses a large gill-cover or operculum,
Fig. 5i8. -Horizontal section supported by several flat bones, and if we lift

through the breathing-organs A x '

of a Teieost. The freeiy-pro- this up the gill-filaments are to be seen aggre-

jecting gills are darkly shaded, - 1 r 1 i IM -11 i

and the course of the breathing- gated into a number of red comb-like gills, the

water indicated by arrows. pi i« ,1 ,1 /- 1

t. Tongue; gi. guiiet. filaments corresponding to the teeth of the

combs. The evolution of the protective gill-
cover has rendered such an arrangement possible.

Many Teleosts also exemplify a special device by which a.
regular flowr of water over the gills is promoted. There are
valvular flaps (inner lips) just within the mouth, and similar flaps
close to the slits behind the gill-covers. When the fish opens its
mouth and dilates its pharynx the latter valves close, so that water
enters from the front only, none passing in under the gill-covers
and through the gill-clefts. As the mouth shuts, its valves come
together, so that all the water that has been taken in is forced
out through the gill-clefts, and is entirely prevented from being
ejected again in front without having done any breathing work.

PROTOCHORDATES

In describing the breathing organs of fishes use has several
times been made of the word ''gill", and it is time to enquire

VERTEBRATES THAT BREATHE IN WATER 389

what is meant by this expression. A gill is an outgrowth from
the body, or it may be a collection of outgrowths, the use of
which is to assist breathing in water. It is, in fact, a means
of increasing the surface through which the dissolved oxygen in
the surrounding water can pass into the blood, while on the other
hand waste carbonic acid from the blood diffuses out into the
water. In an ordinary bony fish each comb-like aggregate of
filaments may be termed a gill in this sense.

In the lowest animals (Protochordates) which have any claim
to be considered vertebrates there are no gills in the sense just
•explained, but the perforated walls of the pharynx offer a suffi-
ciently large surface to serve the purpose of breathing. In the

GILL SLITS OVARIES LIVER ATRIOFORE INTESTINE

Fig. 519. — Lancelet (Amphioxus lanceolatus}
Side view, with internal organs seen by transparency. Semidiagrammatic

Lancelet (Amphioxus), for example, the pharynx is exceedingly
large, and perforated by a great number of oblique clefts (fig.
519), and, as each of these is again divided by cross-bars, the
result is the establishment of a complex basket-work, possessing
a very large surface for exchange of gases between the blood
and the surrounding water. By means of horny supporting rods
the pharynx is stiffened, and the numerous small openings kept
from collapsing. Although the Lancelet is low down in the
vertebrate branch, it must not be imagined that this complicated
arrangement gives any idea of the breathing organs possessed
by the first vertebrates evolved, for the richly -ciliated pharynx
is a current-producing organ which is essential to feeding (see
p. 244), and it has been elaborated in connection with this
important function.

The Ascidians or Tunicates possess a pharynx with side-walls
converted into a basket-work (see vol. i, p. 297), not unlike
that found in the Lancelet, and, as in that animal, its duty is
to cause water to stream in at the mouth, bringing with it food
and the oxygen necessary for breathing. Both in the Lancelet

39°

ANIMAL RESPIRATION

v.b.i

v.n

and in Ascidians the minute organisms and organic particles
which constitute the food would be liable to pass through the
lateral perforations of the pharynx and so wasted, if there were
not some special arrangement to prevent it. But in each of
these cases there is present a groove along the upper side, and
a similar groove along the lower side, along which food is con-
ducted to the gullet, entangled in a
sticky fluid which prevents their escape.
In the Acorn-headed Worm (Bala-
noglossus) (see vol. i, p. 300) the
pharynx is divided by projecting folds
into a lower food-conducting section
and an upper breathing portion (fig.
520). The latter possesses a large
number of pairs of gill-pouches, which
open to the exterior by rounded pores
on the upper side of the body. The
walls of these pouches are supported
by horny rods closely similar to those
which stiffen the side-walls of the
Lancelet's pharynx.

In all these Protochordates — and
the remark is true for vertebrate ani-
mals generally — the pharynx, originally concerned with the diges-
tive function only, has acquired a new use, having been pressed
into the service of breathing. Various devices have been evolved
to prevent these two distinct duties from interfering with each
other, and we have just seen how this is accomplished in the
Lancelet, &c. The use of gill-rakers in Fishes has also been
explained (see p. 387), and we shall have occasion to note in
the sequel the way in which air-breathing vertebrates attempt,
with more or less success, to keep the feeding tract distinct from
the breathing tract.

Fig. 520. — Section through an Acorn-
headed Worm [Balanoglossus] in the region
of the pharynx. This is incompletely divided
into feeding-tube (f.t.} below and breathing-
tube (b.t.} above. The latter communicates
by means of gill-pouches (g.p.} with the ex-
terior. The course of the breathing-water
indicated by arrows, b.c. Body-cavity; d.b.v.
and v.b.v. dorsal and ventral blood-vessels;
d.n. and v.n. dorsal and ventral nerves.

CHAPTER XXXIII

ANIMAL RESPIRATION— NEMERTINES— MOLLUSCS
WHICH BREATHE IN WATER

NEMERTINES

Some authorities are of the opinion that these curious un-
segmented marine creatures, which look like long cylindrical
worms (see vol. i, p. 305), resemble in certain respects the re-
mote stock which has given origin to backboned animals. One
characteristic feature of such animals is, as we have seen, the
possession of a perforated pharynx which has taken on the
function of breathing. Nemertines in all probability breathe to
a large extent by means of their soft skin, but it is also likely
that the pharynx takes part in the same work, although it is
not perforated by gill-clefts. And it is certainly a suggestive
fact that the straight digestive tube of these creatures bulges
out on each side into a series of pouches, resembling in character
the structures which probably gave rise to gill-pouches by acquiring
external apertures (see p. 381).

MOLLUSCS WHICH BREATHE IN WATER

The vast majority of MOLLUSCS of all kinds live in water,
and breathe the oxygen which is dissolved in it. They are
predominatingly marine, but some forms live in estuaries, while
others have migrated into fresh water, and adapted themselves
to the special conditions there existing.

MAIL-SHELLS, &C. (PROTOMOLLUSCA)

The breathing organs are in a comparatively simple condition
in the primitive forms known as Mail- Shells (Chiton, &c.). Here
there is a flattened oval body, protected above by a row of eight

392

ANIMAL RESPIRATION

shelly plates, and exhibiting below a large flat muscular foot, by
means of which creeping is effected. Overhanging the foot,
and running right round the animal, is a flap-like mantle-skirt,

the groove under which
is known as the mantle-
cavity. This shelters on
each side a varying num-
ber of gills (fig. 521),
s each of which consists
of a stalk, on either side
of which are arranged a

INTESTINAL
OPENING

Fig. 521.— Mail-Shell (Chiton] seen from above (A) and below (B).
Note in B the numerous gills on each side

large number of small
flat plates. These col-
lectively present a very
large surface to the surrounding water, and as the gills are of
delicate texture it is easy for the abundant blood which they

contain to get rid of its
waste carbonic acid gas in
exchange for oxygen.

FIN

Fig. 522. — Dissection of a Cuttle-Fish (Sepia) from the back, to
show interior of gill-cavity. The muscular mantle has been cut
through and turned over to the right, and the tentacles have been
cut short: X X (on mantle), two projections which "button" into
two corresponding pits (X X) while water is being ejected through
funnel; K K, kidney openings; Lap. Intestinal aperture. Course
of breathing-water shown by arrows.

HEAD-FOOTED MOL-
LUSCS (CEPHALOPODA)

A Cuttle -Fish (Sepia,
&c.) or Squid (Loligo, &c.)
is far more specialized than
a Mail -Shell as regards
breathing organs. On the
hinder side of its long body
we can easily find a large
gill-cavity, opening below
by a large slit, and probably
equivalent to the posterior
part of the mantle-cavity in
the Mail-Shell, greatly en-
larged and deepened. Two
very large plume-like gills
are contained in the cavity
(fig. 522), and these pro-
bably represent one of the
last few pairs of a Mail-

HEAD-FOOTED MOLLUSCS (CEPHALOPODS)

i. Paper Nautilus. 2. Squid. 3. Egg Capsules of Squid. 4. Common Octopus.

MOLLUSCS WHICH BREATHE IN WATER 393

Shell's gills. It is in fact a case where efficiency has been gained
by the suppression of some of the members of a series (see p. 384),
those remaining becoming larger and more specialized. In an
active rapacious animal like a Cuttle-Fish some special method of
constantly renewing the water in the large gill-cavity is an obvious
necessity, especially as all the waste products of the body (in addi-
tion to carbonic acid gas from breathing) are discharged into it.
We find that the back wall of the cavity is extremely muscular,
and during life contracts at regular intervals, as can readily be seen
in an aquarium specimen, so as to force out the impure water
and the various waste substances. These are expelled through
a muscular tube, the funnel, pure water being admitted by the
large slit already mentioned. As will be explained in another
section, the waste water is ejected with such force through the
funnel that it enables the animal to swim backward with great
velocity. As in so many other cases, various organs of the body
are always liable to have fresh duties imposed upon them, in
addition to their own proper and original work. And as in the
course of time such an organ often gives up the old work entirely,
and devotes itself wholly to the new, we find that the animal
kingdom presents numerous cases of "changes of function".

The breathing apparatus of the Pearly Nautilus {Nautilus
pompilius] is a good deal like that of the Cuttle-Fish, but there
are four gills instead of two, and the funnel is of simpler nature,
consisting of two halves rolled together but not fused.

SNAILS AND SLUGS (GASTROPODA)

The typical SEA-SNAILS and SLUGS (marine Gastropods) agree
to some extent with Cuttle- Fishes, for they have given up the
series of gills which their remote ancestors probably possessed,
and those which remain are sheltered in a comparatively spacious
gill-cavity, into which all the waste products of the body are
discharged. But as a consequence of an extraordinary twisting
round of the upper parts of the body which has taken place, one
result of which is the spiral shape of the shell, this cavity is placed
in front, being so to speak over the shoulders, and opening by
a large slit above the neck region. In some cases two gills are
present, e.g. in the Ormer or Sea-Ear (Haliotis] (fig. 523), and
the waste water makes its exit by a slit in the roof of the gill-

394

ANIMAL RESPIRATION

cavity, the position of which is marked by a row of holes in the
shell. The Keyhole Limpet (Fissurella] possesses the same
number of gills, and the water which has traversed the gill-
cavity makes its way out by the hole at the top of the shell to
which the common name of this form is due. There are two

interesting- relatives of this

TFNTACLES

type which lead up to the
arrangement described. In
one of these (Emarginula)
there is an exit slit at the
front edge of the shell, while
"HA ^ the other (Rimula) there

Fig. 523.— Ormer (Haliotis] seen from above after
removal of shell. The gill-cavity has been opened.

Fig. 524. — Shells of various Sea-Snails

(seen from above)

A, Emarginula; B, Rimula; c, Fissurella; D and E,
successive stages in growth of Fissurella.

is a hole placed a little way from the front (fig. 524). If the
development of the Keyhole Limpet is traced it is found that
there is first of all a slit at the front edge of the shell, then an
opening rather farther back, and lastly the aperture at the top
of the shell which characterizes the adult (fig. 524). This would
certainly appear to be a case of recapitulation, in which the life-
history of the individual epitomizes the family history, and the
two allied forms just described retain throughout life what were
no doubt stages in the evolution of Keyhole Limpets.

Examination of a typical spiral sea-snail, such as the Whelk
(Buccinum) or the Purple- Shell (Purpura), shows that only one
gill is present, one member of the original pair having been
sacrificed for greater efficiency, and this gill is placed very much
to one side (fig. 525). Two gills probably blocked up the gill-

MOLLUSCS WHICH BREATHE IN WATER

395

cavity to such an extent that regular entry and exit of water were

interfered with, while the arrangement which has superseded the

old one enables fresh water to flow up one side of the gill-cavity,

over the single gill, and then down the other side, washing

out the various waste products

to the exterior. And in the

carnivorous forms which have

been taken as examples the

entering water is provided with

a special means of access in

the form of a sort of tube or

" siphon ". The position of

this is marked by a notch in

the shell which, as noticed else-

where (see p. 96), is charac-

teristic of carnivorous snails,

as compared with vegetarian

forms which have no siphon,

and therefore no notch to

lodge it. The difference may

be plausibly explained if we

remember that carnivorous

snails have to be specially

active in order to get a living,

and as activity is associated

with properly -purified blood,

i
We may expect SUCH CreatUreS

r/-> Viow^ m/~it-^ ^ff^riVo Kt-/^o<4-»

to nave more effective breatn-
ing arrangements than their

o o

mOre Sluggish relatives Which

live on vegetable food.

The breathing organs of the Common Limpet (Patella vul-
gata) are in a particularly interesting condition. If we remove
the large conical shell which covers the back of this animal, and
look for a gill-cavity in the position where it is found in a Whelk
or Purple- Shell, we shall readily discover it, though in size it is
comparatively small. On opening this cavity, however, no gill
is to be found, although some not very distantly-related forms, as,
e.g., John Knox's Limpet (Acnuea testudinalis), possess one well-
developed gill in this position (fig. 526). Careful examination

Fig. 525.— Diagram or a Whelk (Buccinum), seen from
above. Shell removed and the roof of gill-cavity sup-
posed transparent i, Mouth; 2, brain-ganglion; 2^, nerve-

cord connecting side.gangiion (above) with foot-gangiion
,(below); 3' ™e of, the tjre" ganglia on the twisfted newe'

loop; 4, gill; 4^, osphradium ; 5, opening of intestine;
6> heart in Pericardium; 8, a gland (purple-gland in Pur-
pura); 9, siphon; 10, 10, foot; n, operculutn.

ANIMAL RESPIRATION

of the floor of the Limpet's gill-cavity shows the presence of a
minute orange-coloured projection on either side, and the position

Fig. 526.— Gill-cavities of John Knox's Limpet (A) and Common Limpet (B) enlarged, as seen from above
with roof removed, i, Intestinal aperture; K K, kidney apertures

and nerve-supply of these prove them to be the vestiges of two
gills which ancestral Limpets no doubt possessed (fig. 526).

How then does a Limpet
breathe? The answer is,
that it has developed new
gills for itself of quite a
fresh kind, in the form of a
large number of little plates
which grow out from the

Fig. 527.— Diagram of Sea-Hare (Aplysia\
seen from above

i, Mouth; 2, nerve-ring with ganglia; 3, one of the
two ganglia on the untwisted nerve-loop; 4, gill, just in
front of which is seen the osphradium ; 5, opening of
intestine; 6, heart in pericardium; 10, loa, right para-
pod; io<5, left parapod folded over back.

Fig. 528. — Diagrammatic cross section of
Hind-gilled Snail (A), and longitudinal section
of Fore-gilled Snail (B). a, Space occupied by
internal organs; bb, shell; cc, mantle; d, foot;
e, gill in gill-cavity.

mantle-skirt that runs right round the body (as in a Mail-
Shell) and overhangs the foot. These mantle-gills are very

MOLLUSCS WHICH BREATHE IN WATER 397

conveniently situated, being washed with sea-water whenever the
tide is up.

So far we have considered the Fore-gilled Sea-Snails, in
which the gill-cavity has been brought right round to the front,
as the result of twisting, and the gill or gills which it usually
contains are therefore in front of the heart, which they supply
with pure blood. But in the Hind-gilled Sea-Snails the upper
part of the body has begun to twist back again (in the direction
of the hands of a watch) so that the gill-cavity has shifted from
the front to the right side of the body. This is the case, for
example, in the curious slug-like Sea- Hare (Aplysia), where the
laterally-placed gill-cavity contains a single gill which is now
placed rather behind the heart (figs. 527 and 528). This group
of Molluscs also includes the Sea - Slugs proper, which have
struck out a line of their own, and have lost shell, gill-cavity,
and typical gills. Some of them, however, have been enterprising
enough to grow new
gills, just as the Limpet
has done. Of this a good
example is the Sea-
Lemon (Doris), in which
a beautiful circlet of
feathery gill-plumes is
situated far back on the
upper side, surrounding
the opening of the intes-
tine (fig. 529). Although
these have no gill-cavity to shelter them they are well-protected,
for they can be drawn back into a groove until no trace of them
can be seen from the outside.

Other Sea-Slugs (as Elysia) are content to breathe with the
general surface of the body, and this method appears to purify
the blood sufficiently in a large number of thin-skinned animals
belonging to diverse groups, especially when they are of small
size. This is partly dependent on the fact that the smaller the
animal the larger the surface of the body relative to its bulk,
besides which none of the organs are very far removed from the
surrounding medium, and consequently have but little difficulty
in exchanging their waste carbonic acid gas for oxygen.

Fig. 529. — Side view of Sea-Lemon (Doris)

398

ANIMAL RESPIRATION

BIVALVE MOLLUSCS (LAMELLIBRANCHIA)

Bivalves possess well -developed breathing organs, as may
readily be seen by even a superficial examination of any common
form. If we take, for example, a Mussel, Cockle, or Oyster, we

shall find the body covered by a pair of
strong calcareous plates, which are the
halves of the bivalve shell, and are
placed on the right and left sides of the
body. These valves cover, and indeed
are chiefly secreted by, two flaps of the
body -wall, constituting together the
mantle-skirt. If a human being were
clothed in a coat so large that its sides
completely covered his body, head, and
limbs, he would serve as a rough model
of the arrangement — that is, if one could
imagine the coat to be a part of him
and not merely an investment. These
two large mantle-flaps of a bivalve do
Fig. 53o.-Freshwater Mussel (Anodonta] & considerable amount of the work of
opened and seen from below breathing, and if one of them be lifted

a, Position of mouth; bb, adductor muscles -, , 1M . 77 -ill

(which keep shell closed) cut through; c, Up a large platC-llKC gill Will DC SCCtt
mantlejobe; d, labial palps or feelers; ,. foot; beneath ft Jn any Qne of ^ three CQm_

mon types named (fig. 530). These

gills, the shape of which has suggested the name of the group
(Lat. lamella, a thin plate; Gk. branchia, gills), share the work of
breathing with the mantle -flaps, and are usually regarded as
equivalent to the two plume -like gills of a
cuttle-fish or ormer. We have seen (see
p. 248) that bivalves are in some respects
degenerate, and depend for food upon ciliary
Fig. 531.— Freshwater MUS- currents, which carry minute organisms to the
mouth. Both mantle -flaps and gills are richly
covered with cilia, and act as current-pro-
ducing organs, thus enabling feeding and
breathing to go on, and also providing for the
removal of products of waste when the animal is buried in sand
or mud, with only the hinder end projecting (fig. 531). We

sel (Anodonta] embedded
obliquely in mud, with hinder
end projecting. The arrows
indicate the currents of water
which enter and leave the
mantle-cavity

MOLLUSCS WHICH BREATHE IN WATER 399

may therefore correlate the large size and great complexity of
the gills of freshwater mussels, &c., with the fact that they do
double duty, like the pharynx of a Lancelet or an Ascidian
(see p. 389).

The plate-like gills of a Freshwater Mussel are remarkably
complex, constituting the last term of a series to which arrange-
ments found in other bivalves lead up. The simplest case is
that of certain forms {Nucula, &c. ), where either gill consists
of an axis bearing a double series of little flattened outgrowths.
An advance upon this is seen in the Saltwater Mussel (Mytilus),
for here the little outgrowths are represented by long threads,
which turn up at their ends owing to the limited size of the
gill -cavity. Adjacent threads are connected together by the
interlacing of long cilia. Each gill of a Freshwater Mussel
(fig. 530) consists of an outer plate and an inner plate. These
have arisen by fusion of the outer and inner sets of filaments,
which simply interlock in a Saltwater Mussel.

A few bivalves (Cuspidaria, &c.) have modified in another
way. The gill on either side is only represented by a horizontal
perforated plate of muscular nature, which divides the gill-cavity
into upper and lower compartments. In these forms the mantle-
lobes must do most of the breathing work.

CHAPTER XXXIV

ANIMAL RESPIRATION— JOINTED-LIMBED ANIMALS
WHICH BREATHE IN WATER

Two classes of Arthropods exemplify adaptations for breathing*
in water, i.e. Crustaceans and K ing-Crabs. And as in other
groups of animals, the higher members are more specialized in
regard to their breathing organs, as well as in other respects.

CRUSTACEANS (CRUSTACEA)

The HIGHER CRUSTACEA, such as Lobsters, Crayfish, Prawns,.
Shrimps, and Crabs, possess large and complex gills, all of which
are borne on the thorax, or region which succeeds the head. The
Common Lobster (Homarus vulgaris] is a convenient first example,
and it may be premised that the eight pairs of limbs borne by its
thorax are, beginning in front, three pairs of foot-jaws which help
to tear up the food, the huge pincers, and four pairs of walking-
legs (see vol. i, pp. 403-405). On first inspection no gills at
all are visible, and this is because a special shelter has been pro-
vided for these delicate structures. There is, in fact, a spacious
gill-chamber on each side of the body, covered by a large gill-cover
extending down to the bases of the limbs. There is, however, a
slit by which water can enter behind and below, and go out in
front. On removing this protective covering (which recalls the
gill-cover of a bony fish as regards its use) a number of large
gills are seen (fig. 532). Each of them is like a bottle-brush (to
use Huxley's simile), and consists of a central axis beset with
numerous threads. On the outside are six limb-gills, attached to
the bases of the last two pairs of foot-jaws, the pincers, and the
first three pairs of walking-legs. On turning these back ten joint-
gills are seen, attached in pairs to the junctions between the limbs
and body from the last foot-jaws to the third pair of walking-legs,.

400

JOINTED-LIMBED ANIMALS WHICH BREATHE IN WATER 401

inclusive. If these joint-gills are folded back in their turn, four
side-gills become apparent, so called because they grow out from
the side wall of the body. They correspond to the last four seg-
ments of the thorax, which bear the four pairs of walking-legs.
Each limb-gill is associated with a plate-like outgrowth (epipod)

Anlcnnu/c

FJ^. 532.— Gills of Right Side of Lobster (Homarus vulgaris), reduced.

The gill-cover has been removed, the pincers (i) and four walking-legs (n-v) cut short: 1-7, segments of abdomen
and their appendages; aaaa, limb-gills (two last turned down); bb, joint-gills

from the limb to which it belongs, and the chief use of these
plates appears to be to help to keep the gills separate, so that
water may pass between them. From what has been said it
will be seen that there are twenty gills in all on each side of
the body, and this is most easily understood by presenting the
facts, for one side, in a tabular form.

GILL-FORMULA OF LOBSTER (SIMPLIFIED AFTER HUXLEY).

Segments of Thorax. Limbs. Limb-gills. Joint-gills.

Side-gills.

ist foot-jaw. o o

2nd „

3rd „

Pincers.

ist leg.

2nd „

3rd „

4th „ oo

VOL. II.

+ 4-20

402 ANIMAL RESPIRATION

The simplest way of explaining the many variations that are
to be found as regards number among the gills of various Higher
Crustaceans is to suppose that they are reductions of various
extent on a primitive condition, in which each of the eight rings
or segments making up the thorax carried four gills on each side
of the body, i.e. one limb-gill, two joint-gills, and one side-gill,
giving an original total of thirty-two. If this be the case, the
Lobster has lost twelve gills each side, those that remain having
become more efficient. This is the same principle on which the
gills of fishes and of molluscs have already been explained (see

PP- 393-394).

The forty gills of a Lobster present a very large surface for
purification of the blood, but they would be of little use if there
were not some efficient arrangement for renewing the water in
the enclosed gill-chambers. There are no cilia, as in a Lancelet
or Mussel, to cause a steady flow of water, and other means have
therefore to be employed. The movements of the limbs, to which
limb-gills and joint-gills are attached, must do something in this
direction, and the more quickly a Lobster moves about the better
they act. They are not able, however, to do more than stir up
the water, without producing a steady current in a definite direc-
tion. Such a current does exist, the impure water flowing out
steadily from the front of each gill-cavity, and careful observation
of an aquarium specimen shows how this is brought about. Just
at the front of each gill-chamber something will be seen in con-
stant movement, and examination of a dissected Lobster shows
that this something is the edge of a boat-shaped plate, the baler,
which lies within the gill-chamber in front of the gills (fig. 532),
and constantly scoops out the water, hence its name. The plate
is part of the last of the three jaws belonging to the head (second
maxilla), and during life these jaws are in constant movement from
side to side, which of course keeps their baling plates working.
Another very interesting device also deserves notice. It is
extremely important for parasites and particles of dirt to be kept
out of the gill-chamber, and this is managed to some extent by
tufts of long hair-like bristles which are attached to the bases of
some of the limbs, and strain the water as it enters the gill-
chamber.

The Freshwater Crayfish (Astacus fluviatilis) has undergone
greater reduction than the Lobster as regards its breathing organs,

JOINTED-LIMBED ANIMALS WHICH BREATHE IN WATER 403

for it possesses only eighteen gills each side, there being, for
example, only one side-gill, the last of its series. But in front
of this are two little threads placed where other side -gills are
situated in the Lobster, and these are no doubt vestiges of such
gills, which Crayfishes once possessed but have now all but lost.

A very large reduction has taken place in the Common Prawn
(Palczmon serratus), for it has but eight gills each side, i.e. one limb-
gill, two joint-gills, and five side-gills. In spite of the reduction
it will be seen that it possesses one more of the last kind than the
Lobster. It is also to be noted that the Prawn's gills, instead of
being like bottle-brushes, are plume-shaped, consisting of a central
axis bearing two sets of leaflets, much as in the typical gill of a
Sea-Snail (see fig. 523, p. 394).

Some of the Prawns and Shrimps use the first walking-leg for
cleaning the gills, pushing it into the front end of the gill-chamber
for this purpose.

The Common Hermit-Crab (Pagurus Bernhardus) cleans its
gills by means of the last pair of walking-legs, which are small,
and, like the pair in front of them, find their chief use in holding
on to the shell in which their owner lives.

Crabs constitute the most highly specialized of all the groups
of Higher Crustaceans, and exhibit a large amount of reduction
and modification in their breathing organs. The most typical
species, of which we may take the Edible Crab (Cancer pagurus]
as a type, are shore-forms, which spend a good part of their time
out of water, so that special provision is necessary to prevent
their gills from drying up by exposure to the air. On examining
the type mentioned we shall find that the gill -cover is in such
close contact with the bases of the limbs that water cannot enter
the gill-chamber from behind or below, as in the Lobster and
its allies. There is a special aperture in front for this purpose,
which can be closed when necessary by a sort of door or valve
constituted by a part of the last pair of foot-jaws. Waste water
is scooped out by the baler through two other openings situated
still farther to the front. On opening one of the gill-chambers
seven large plume-gills are at once visible, of which five are joint-
gills, while the other two are equivalent to the first two side-gills
of a Lobster. There are also two limb-gills belonging respectively
to the second and third foot-jaws. There is no gill attached to
the first foot-jaw, but it possesses a large plate or epipod (belong-

404

ANIMAL RESPIRATION

ing to a vanished gill), which is long and curved and can be
moved over the outer surface of the gills, serving apparently
to keep them clean (fig. 533). The epipods of the second and

third foot-jaws project back under
the gills as " churning rods", help-
ing to maintain the circulation of
water.

In the Mantis- Shrimps (Squilla,
&c.) the tail or abdomen is of rela-
tively large size, and tufted gills
are borne by the outer parts of its
limbs (fig. 534). It is a much
simpler arrangement than those so
far described.

Sessile-eyed Crustacea, in which
the eyes are devoid of stalks, are
divided into Amphipods, flattened
from side to side, and Isopods,
flattened from above downwards.
In the former group are included
the little Sand- Hoppers (Talitrus,
&c.) and their allies, in which the gills are simple plates carried
on the inner sides of the limbs of the thorax (fig. 534). The
attenuated Skeleton- Shrimps (Caprelld] practically consist of head

Fig- 533-— Gills of a Crab

The roofs of the gill-chambers have been removed
and four gills cut short on the left side: CL.R., clean-
ing-rods (epipod) of first foot-jaws, the left one is cut
short; CH.R., churning rods (epipods) of second and
third foot-jaws.

Fig. 534.— Gills of Mantis-Shrimp and Sand-Hopper

A, Cross section through tail of a Mantis-Shrimp (Squilla), showing a gill on one side and a gill-axis on
the other. B, Cross section through thorax of a Sand-Hopper (Gammants], showing a pair of gills.
BK.P., Brood-pouch; D.GL., tubular digestive glands; HT., heart; INT., intestine; M.M., Muscles; N.C.,
nerve-cord; ov., ovaries.

and thorax only, the abdomen being reduced to a mere stump.,
The thorax is made up of eight rings or segments (of which the.
two first are fused with the head), which is the typical number

JOINTED-LIMBED ANIMALS WHICH BREATHE IN WATER 405

for a Higher Crustacean, and a pair of large rounded gills are
borne on the fourth and fifth of these.

In Isopods, such as the Sea- Slaters (Idotea, &c.) and the
Water Wood- Louse (Asellus\ the gills are delicate plates borne
on the inner sides of the ab-
dominal limbs, the outer parts
of which protect them to some
extent.

Mud- Shrimps (Neb alia, &c.)
breathe by means of eight pairs
of flattened limbs borne on the
thorax. These are protected by

a large thin Shield, Which grOWS Fig" SSS-Mud-Shrimp (Nebalia}, enlarged

frnm thf* VlParl e\\Tf>r tViic Left half of shield cut away. The letter o in THORAX
neaa OVer tniS is piacedon the cleaning filament of the second jaw.

part of the body. The second

jaw on each side bears a flexible whip-like filament, by means ol

which the breathing-limbs are cleaned (fig. 535).

Many of the LOWER CRUSTACEA are of small size, and breathe
entirely or largely through the general surface of the body,
wherever the protective layer (cuticle) that covers the skin is
thin enough to permit of this. Special gills may, however, be
present in some cases, as in the larger members of the Leaf-footed
Crustacea (Phyllopods) such as Apus and Branchipus. These
possess very numerous delicate flattened limbs, each of which
carries a soft rounded gill. In Apus a protective shield grows
back from the head (as in a Mud- Shrimp) and covers a large
part of the body, and the inner side of this shield probably helps
in the work of breathing. The small Phyllopods known as Water-
Fleas (Daphnia, &c.) are distinguished by the shortness of the
body, which, except the head, is enclosed in a bivalve shell quite
comparable in nature to the shield of Apus. There are a few
pairs of flattened limbs, without gills, and these by their constant
movement keep up a stream of water between the halves of the
shell, the inner surface of which shares with them the work of
breathing.

The little Mussel-Shrimps (Ostracods) breathe much in the
same way as Water- Fleas, but the bivalve shell is of greater
relative importance, and encloses head as well as trunk. The
limbs are narrow and specialized, and do not present the large
breathing surface possessed by the leaf-shaped appendages of

406

ANIMAL RESPIRATION

Phyllopods. The second jaw belonging to the head (first maxilla)
is provided with a large fan-like plate which by its movements

helps to maintain a stream of water
through the space between the halves of
the shell (fig. 536).

In Barnacles (Cirripedes) the long
tendril-like feet which fish for food (see
p. 254) probably assist in breathing,
Fig. 536.-A Mussei-shHmp (c^ris), which is also partly carried on by the

enlarged. ANT. i, Antennule: ANT.Z, an- /* .T 1 J .f 1 * 1* 1

tenna; MND., mandible; MX.I, first maxilla SOlt lOlU OI SKin tftat ImCS tilC
(notice the large fan-plate) ; MX. 2, second 1_1at^c nn ^1VU^r C1'J~ Tn A /-r»t-i

maxiiia. plates on eiin< siae. in /\con

nacles (Balanidae) there is a folded pro-
jection between each of these flaps and the bases of the limbs,
which probably acts as a gill.

KING-CRABS (XIPHOSURA)

The curious King- Crabs (Limulus) that shuffle about on
the surface of mud, devouring worms, &c., are protected by

strong shield -armour, through
which breathing cannot take
place. Specialized gills are there-
fore present, and are well pro-
tected, as the habits of their
owners would otherwise render
them liable to be soiled with
mud. On examining the under
side of a King -Crab, a broad
plate, the gill-cover (operculum),
will be seen behind the last pair
of walking-legs (fig. 537). There
is good reason for thinking that
this plate has been formed by the
fusion of two flattened limbs.
Behind it may be observed the
projecting edges of the five pairs
of abdominal limbs, which also
are plate-like, and bear on their upper sides (so as to be well
protected) a large number of delicate gill-folds (fig. 537), arranged
somewhat like the leaves of a book, and offering a large surface

Fig. 537. — King-Crab (Limulus], reduced
On the left the under side is represented and on the
right an abdominal limb with its gill-folds, i, Chelicerse;
2-6, legs. The mouth is seen as a darkly-shaded slit
between the bases of the legs; 7, operculum.

JOINTED-LIMBED ANIMALS WHICH BREATHE IN WATER 407

to the water which surrounds them. The gill-cover and the gill-
bearing limbs can be moved by special muscles so as to renew
the breathing-water from time to time.

CHAPTER XXXV

ANIMAL RESPIRATION— LOWER INVERTEBRATES WHICH
BREATHE IN WATER

SEGMENTED WORMS (ANNELIDS)

The most interesting members of this group, so far as the
present section is concerned, are to be found among the Bristle-
Worms (Chaetopoda), and after first considering these it will be
necessary to add a little regarding Leeches (Discophora).

BRISTLE-WORMS (CH^TOPODA)

The marine Bristle- Worms (Polychaetes) include both actively
carnivorous forms which have no fixed abode, and also tube-
dwellers, although there is no sharp boundary between the two
sub-groups. The skin is in all cases sufficiently thin to be of
use in breathing, and the surface offered for this purpose is in-
creased by the presence of numerous pairs
of hollow foot-stumps, the organs by which
burrowing, crawling, and sometimes swim-
ming are effected. There may, however, be
special respiratory organs on the upper side
of the body, and these not infrequently take
the form of branching gills. In certain flat-
tened forms (Polynoids) there is a double
series of thin scales (elytra) on the back, and
these would appear to answer the same
purpose (fig. 538). Such scales may be visible
externally, but in the Sea- Mouse (Aphro-
dite) they are enclosed in a sort of gill-chamber, which is roofed
over by a felt-work of minute bristles cemented together.

The Lugworm (Arenicola\ which lives in a burrow in the
sand, is a sort of transitional case between those marine bristle-

Fig. 538.— Scale-Worm (Poly-
.noe). Front end seen from above,
to show breathing-scales.

408

LOWER INVERTEBRATES WHICH BREATHE IN WATER 409

Avorms which rove from place to place and the greatly specialized
tube-dwellers. It possesses delicate branching gills along part
of its upper surface (fig. 539). But in cases where there is a
definite dense-walled tube of lime, or horny matter,
or sand-grains and the like cemented together, it is
clear that the trunk-region is unfavourably situated
for breathing, whether by its general surface or by
means of special outgrowths. It is not, therefore,
surprising to find in such cases that the head bears
gills, generally in the shape of a pair of large feather-
like plumes covered with cilia (Terebella, Sabella,
Serpula, &c.) (fig. 540). Currents of water are set
up by the action of the cilia, and these not only con-
stantly renew the
water necessary for
breathing, but also
wash away waste-
products, and keep
up the supply of food
(see p. 258). It is
interesting to note
that these structures
present another in-
stance of " change of
function ", being, in
fact, old organs put to
a new use, and cor-
responding to a pair
of little projections

rum], reduced.
To show the fea-
thery gills.

phcato- (pa]pS) possessed by

Fig. 540.— A Tube- Worm (Terebella}, showing
gill-plumes on head

the head of an ordi-
nary marine worm

where they are concerned with touch, and perhaps other senses

as well.

LEECHES (DiscoFHORA)

In most Leeches the soft moist skin is sufficiently effective as
a breathing organ without the aid of gills, and it is very richly
provided with blood-spaces, many of which are unusually near
the surface. There is nothing very extraordinary in this, for

4io ANIMAL RESPIRATION

breathing organs of all kinds must of necessity be closely related
to the blood-system.

There is, however, a marine leech (Branc hellion), living as a
parasite upon the Torpedo, which possesses gill-tufts reminding
one of an arrangement common among marine bristle-worms.

SIPHON-WORMS (GEPHYREA) AND WHEEL-ANIMAL-

CULES (ROTIFERA)

SIPHON-WORMS do not as a rule possess special gills, but
breathe by means of the skin. In forms like the Bristle-Tail
(Echiurus) and Bonellia (see p. 150), where there is a projecting
proboscis in front of the mouth, it is probable that
this organ is concerned with breathing as well as with
feeding. In the sand -swallowing Siphon -Worm
(Sipunculus) (see p. 259) the mouth is surrounded by
a horse-shoe-shaped fold with a folded edge, and it
is likely that breathing is one of the uses of this fold.
Quite a different arrangement is found in an allied
form (Priapulus) in the shape of a deeply-lobed
appendage at the hinder end of the body, and which
is almost certainly to be regarded as a breathing
organ (fig. 541). Siphon- Worms also breathe in part
by means of the digestive tube.

Fig. 541.— Priapu- r^i . ,-rr A ...

lus (reduced). RESP., 1 he minute WHEEL - ANIMALCULES, like many

RESP,

other small animals, do not require anything ela-
borate by way of gills, and breathe through the
general surface of the body. The ciliated wheel-organ at the
front end sets up currents in the surrounding water, which no
doubt, as in so many other cases, not only bring food but also the
oxygen necessary for respiration. In those Wheel- Animalcules
which live in cups or tubes the wheel-organ is particularly large and
often complicated, reminding us of the large complex gills which
are borne upon the heads of certain tube-worms (see p. 409).

MOSS-POLYPES (POLYZOA) AND LAMP-SHELLS

(BRACHIOPODA)

The members of both these groups have already been adduced
as instances of animals which feed by means of ciliary currents

LOWER INVERTEBRATES WHICH BREATHE IN WATER 411

(see p. 243), the current-producing organs in either case being a
more or less complex plume in the neighbourhood of the mouth.
These organs are no doubt also effective for breathing purposes.
In Moss- POLYPES something of the kind is clearly necessary, for
although in these colonial animals each individual is of small
size, and might perhaps be expected to breathe by means of the
general surface, a large part of this is prevented from doing
respiratory work. For every member of the colony is to a great
extent enclosed in a horny cup, which is an effectual hindrance
to exchange of carbonic acid gas for oxygen, and the soft part of
the body which projects from the cup would not give a sufficiently
large breathing surface were there not special outgrowths from it
(see p. 261).

LAMP-SHELLS are solitary animals of much larger size than
the individual members of Moss- Polype colonies, and it is there-
fore not astonishing that there should be outgrowths from their
bodies which serve as gills, though (as in Moss- Polypes) these
outgrowths play a double part. Nor must it be forgotten that
the body of one of these animals is covered and protected by
a firm bivalve shell, which in one sense diminishes the surface
available for breathing. This, however, is compensated by the
fact that each half of the shell is lined by a thin flap of the
body-wall which takes part in the work of respiration. Although
Lamp- Shells are but very distantly related to Bivalve Molluscs,
there is a curious similarity between the two groups as regards
feeding and breathing (see p. 248). This is only one of very
many cases in the animal kingdom where more or less similar
organs have been independently evolved in quite distinct groups,
which have had, so to speak, the same physiological problem to
solve.

ECHINODERMS (ECHINODERMATA)

This large, important, and very sharply limited group or
phylum of the animal kingdom includes Sea- Lilies (and Feather-
Stars), Sea-Urchins, Star-Fishes, Brittle-Stars, and Sea-Cucumbers,
besides other forms which have long been extinct. Two char-
acteristics of these animals are especially noteworthy, as they
largely influence the nature of the breathing organs and mode of
breathing. They are: (i) A strong tendency to develop a firm
protective armour, consisting of limy plates in the skin, often

4I2

ANIMAL RESPIRATION

united by their edges, and frequently bearing spines. (2) The
possession of a set of tubes collectively known as the " water-
vascular system ", this name having been given because the
system is connected with the exterior, for the purpose, it would

t.f.

-mb.

Fig. 542. — Mouth-area of a Sea-Urchin (Echinus esculentus], enlarged

*#., Mouth with five teeth; mb., membrane surrounding mouth; o.g., an oral gill; /., specialized tube-foot;

t.f., ordinary tube-feet.

appear, of taking in sea-water. The most important parts of the
water- vascular system are a ring round the gullet, one or more
canals by which this is connected with the outside of the body,
and five branching tubes which it gives off to the "arms", rays,
or equivalent parts of which the body largely consists.

INFLUENCE OF THE SKELETON ON THE DEVELOPMENT
OF BREATHING ORGANS

It is quite evident that the possession of a firm external
skeleton must more or less prevent breathing by means of the

LOWER INVERTEBRATES WHICH BREATHE IN WATER 413

general surface of the body, the limitation being greatest where
the skeleton is best developed and most continuous.

CRINOIDS. — In a Feather-Star (Comatula) the body consists of
a central cup, from the margins of which five branching feathery
arms grow out. One side of the cup and the corresponding sides
of the arms are strengthened by a continuous set of calcareous
plates, through which breathing cannot take place. But the other
side of the body, in the centre of which the mouth is situated, is
covered by leathery skin, and it is necessarily this side which does
the work of respiration, chiefly by means of special outgrowths, as
will be explained later on. In Sea- Lilies it is usual for both sides
of the body to be supported by firm plates, between which the soft
breathing organs project.

In SEA-URCHINS (Echinoids) the body is supported and pro-
tected by a firm test, composed of numerous calcareous plates
united by their edges, and thus the area of the general surface
available for breathing is very much reduced. The mouth, how-
ever, is situated in the middle of a soft membrane which no doubt
partly performs this function, and its efficiency is greatly increased
by the presence of
five pairs of branched
oral gills which pro-
ject from it (fig. 542).
The digestive tube is
also specialized to
assist in respiration,
for part of the water
which enters the
mouth with food is
conducted along a
narrow tube (''si-
phon") that branches
off from the gullet,
and later on opens
into the intestine.

In ordinary STAR-
FISHES (Asteroids) the skeleton is not so complete as in the last
group, so far as the upper surface and the sides are concerned,
and numerous thin plates are left, from which small branching
gills are protruded (fig. 543).

fig. 543. — Cross section through Arm of Star- Fish enlarged

g, Gill; pppp, calcareous plates (hard parts are shaded obliquely). Two

tube-feet are seen below

4i4 ANIMAL RESPIRATION

BRITTLE- STARS (Ophiuroids) are enclosed in very complete
scale-armour, so that special provision for breathing is a necessity.
There is a conspicuous swelling placed between the bases of
every two adjacent arms, and each of these contains a couple of
pouches opening by slits to the exterior. The ten pouches have
a ciliated lining, by means of which sea-water is made to flow
continuously through them for breathing purposes. And in some
cases the slit-like opening is divided into two holes, one of which
admits sea-water to the pouch, while the other serves as a means
of exit.

SEA-CUCUMBERS (Holothurians) are leathery elongated forms
in which the external skeleton is reduced to detached plates
imbedded in the skin, so that a large part of the general surface
is available for breathing. In many of these creatures there is
a curious internal arrangement by which a great deal of the work
of respiration is effected. There are here, in many cases, two
large branched respiratory trees which open into the intestine
(fig. 545), through the external opening of which water is alter-
nately taken in and squeezed out, so that the large surface pre-
sented by the lining of the trees is constantly bathed with fresh
water, enabling exchange of carbonic acid gas for oxygen to be
readily carried on.

THE RELATION OF THE WATER- VASCULAR SYSTEM

TO BREATHING

It is extremely probable that the water- vascular system as
we now find it was first evolved as a means of breathing, for in
all cases its radiating branches bear very numerous slender pro-
jections with thin walls, collectively presenting a very large
respiratory surface, making up for the area rendered useless for
this purpose by the development of firm calcareous plates in the
body-wall.

CRINOIDS. — A typical case is presented by the Feather-Star
(Comatula), in which five ciliated grooves run outwards from the
mouth to send branches along the arms, and all their subdivisions.
As described elsewhere, the small particles and organisms which
constitute the food are conducted along these grooves to the
mouth (see p. 265). Examination of any part of one of the
arms shows that the food-groove which runs along it is flanked

LOWER INVERTEBRATES WHICH BREATHE IN WATER 415

on either side by very numerous groups of pointed structures,
which are branches of the water-vascular system and act as gills.
This affords another instance of water-currents produced by ciliary
action having to do double duty, by bringing with them both food
and oxygen. Breathing is effected similarly in the fixed Sea-
Lilies, which are the most typical members of the group to which
the Feather- Stars belong.

The delicate outgrowths just described are also of use as sense
organs.

In ordinary STAR- FISHES (Asteroids) the water- vascular system,
though retaining its uses in regard to breathing and sensation,
has also acquired a new function, for it is here the means
of locomotion. The remote ancestors of these creatures were
almost certainly fixed forms (as Sea- Lilies still are), and when
these became free some means of moving about had to be evolved,
one solution to this problem being found in the way indicated.
On the under side of a Star- Fish five broad grooves are to be
seen, radiating from the mouth, and protruding from these are
numerous tube-feet, equivalent to the delicate projections which
flank the food-grooves in a Feather- Star. The walls of these
tube-feet are sufficiently thin to act as gills, though, as we have
seen, they are not the only, nor probably the chief, organs of
respiration in this case (see p. 413). ,,

SEA-URCHINS (Echinoids). — In an ^BB
ordinary regular Sea -Urchin (see vol. i,
p. 456) of spheroidal shape there are
tube-feet comparable to those of a Star-
Fish, but in this case arranged along five
bands which stretch from one pole of
the sphere to the other. They can be
protruded beyond the tips of the spines
so as to enable the animal to walk or
climb, and they share the work of breath-
ing with the oral gills and the soft mem- ™s tube-feet specialized as gins and

0 arranged like the petals of a flower.

brane surrounding the mouth (see p. 413).

Many of the Sea-Urchins, however, are of " irregular" shape,
being more or less flattened and markedly two-sided. One result
of this has been that the tube-feet present on the upper half of
the animal have become useless as locomotor organs. But they
have not been allowed to remain idle, for their value as breathing

4*6

ANIMAL RESPIRATION

organs has increased and they have become more or less specialized
gills (fig. 544), branched in some cases so as to give a larger
surface.

The structures which correspond to tube-feet in BRITTLE-STARS
(Ophiuroids) project from the sides of the arms, and are of no
use for locomotion. They probably assist in breathing to some
small extent, but their chief use is to act as sense organs.

Most of the SEA-CUCUMBERS (Holothurians) are provided with
tube-feet which help the other breathing organs present, but they

ov.

R.T.

W.V.R.

CUV.

R.W.V/ \R.T

Fig. 545. — Dissection of Sea-Cucumber (diagrammatic)

The water-vascular ring (W.V.R.), bearing stone-canals (ST.C.) and Polian vesicles (P.V.), sends branches to
the tentacles (TENT.), and gives rise to five longitudinal radial vessels (R.W.V.) which bear tube-feet not
shown in figure) ; R.T., R.T., the two respiratory trees opening into CLOACA; CUV., Cuvierian organ, from
which sticky defensive threads are ejected.

are not so important in this respect as the circlet of tentacles at
the front end of the body, which also contains branches of the
water-vascular system (fig. 545).

ZOOPHYTES (CCELENTERATA)

The Hydroid Zoophytes, Jelly-Fish, Sea-Anemones, Corals,
and Comb- Jellies which make up this large subdivision of the
animal kingdom breathe by the general surface of the body, and
the mouth is often surrounded by one or more circlets of tentacles,
which, while their main use would appear to be to secure food
(see p. 156), also largely increase the area over which breathing
can take place. This is particularly necessary in those cases
where there is a firm skeleton by which more or less of the
surface of the body is prevented from taking part in respiration.

LOWER INVERTEBRATES WHICH BREATHE IN WATER 417

In the Organ-pipe Coral (Tubipora musica\ for instance, each
member of the colony lives in a tube, from which only the mouth-
end can be protruded, and it is the external surface of this end
which probably does most of the breathing. The eight broad
feathery tentacles which surround the mouth largely increase the
available area, and they are beset with cilia which set up currents
by which the water in contact with them is constantly renewed.

Examination of fig. 416, p. 162, which represents one of the
free -swimming Compound Jelly -Fishes (Physophora), will show
that the numerous and diversely-shaped members of which the
colony consists present collectively a very large external surface
over which breathing may take place. At the top is a float, below
this a stalk-like part on which are arranged numerous swimming-
bells, and under these come a circlet of leaf-shaped structures,
which cover feeding individuals, from which numerous long
branched fishing-lines trail in the surrounding water.

It must further be remembered that the great diversity of
animals included in the group of zoophytes are to all intents
and purposes living stomachs of more or less complexity, and,
since a great deal of water is taken in with the food, the lining
of the large internal digestive space is able very materially to
help on the respiration. Indeed it may be remarked generally
that in the lower groups of the animal kingdom division of
physiological labour is not effected to anything like the same
extent as in the higher groups.

In some of the Sea -Anemones
there is a special arrangement pro-
moting very greatly the internal
breathing above described. For, as
in all zoophytes, the mouth is not
precisely equivalent to the aperture
so named in, say, an earth-worm, for

i r i , . /- Fig. 546.— Diagrammatic vertical section of

It SerVeS nOt Only tOr the taking in Ol a Sea-Anemone, showing ciliated grooves of

food, but also for casting out such por- ^ef^^^^^^^
tions of this as have not been digested.

It is of slit-like shape, and leads into a gullet along which run a
couple of ciliated grooves, one beginning at each corner of the
mouth. Except when large prey is being swallowed or large
undigested fragments passing out, the sides of the gullet are in
contact and the central part of the mouth-slit closed. The ciliated

VOL. II. 59

4i8 ANIMAL RESPIRATION

grooves are thus converted into two ciliated canals, in one of
which an inward current of sea-water is maintained, while the
other is traversed by an outward current (fig. 546). The in-
flowing water carries with it not merely small organisms and
particles which serve as food, but also dissolved oxygen which
can be used for internal respiration.

SPONGES (PORIFERA) AND ANIMALCULES (PROTOZOA)

As explained in the preceding section, the body of a simple
Sponge is shaped like a cup or vase with walls perforated by
numerous holes (see fig. 265). By means of ciliary action sea-
water is caused to stream continually through these holes into
the central cavity, and thence to the exterior by the opening of
the cup. In this way the animal, in spite of the fact that it is
fixed, obtains an abundant food-supply together with plenty of
oxygen, while at the same time all the products of waste are
swept away. In the more complex Sponges, which are to a
great extent colonial, the body is traversed by a labyrinth of
canals, parts of which are ciliated, but the feeding and breathing
arrangements are essentially the same as in the simple forms.
Although no doubt the outer surface of a Sponge helps in
breathing, it may be said that this function is chiefly performed
by the internal surface, the area of which is greatly augmented
when the canal-system is complex.

ANIMALCULES (PROTOZOA)

The minute size of these simplest animals renders specialized
breathing organs unnecessary, since the outside of the body offers
a sufficient surface for exchange of gases between the substance
of the animal and the surrounding medium. In such cases as
that of the Proteus Animalcule (Amceba) there is not even an
external membrane to hinder diffusion of carbonic acid gas out-
wards, and oxygen inwards, and even where such an investment
is present it is exceedingly thin. If, as an example of a fixed
form, we take the stalked Bell Animalcule ( Vorticella) it may be
noted that the broad end of the body is provided with cilia, the
currents set up by which provide both food and oxygen as in so
many other cases.

LOWER INVERTEBRATES WHICH BREATHE IN WATER 419

In spite of what has been said it would appear that Protozoa
also breathe to some extent by means of internal surfaces, for
water is taken in with each portion of food, and this mixture of
food and drink lies in a temporary hole (food vacuole) within
the living substance of the body, which no doubt takes up dis-
solved oxygen from the watery part of it. The body also contains
one or more clear fluid-filled spaces (pulsating vacuoles) which
alternately increase and diminish in size, and have been proved
in some cases to communicate with the exterior. It is usually
held that this is a sort of pumping arrangement, of respiratory
nature, whereby pure water containing plenty of dissolved oxygen
is taken into the body, and impure water containing waste pro-
ducts ejected to the exterior.

CHAPTER XXXVI

ANIMAL RESPIRATION— BACKBONED ANIMALS WHICH

BREATHE IN AIR

We have now seen that animals which breathe in water may
use for this purpose various surfaces of the body, both external
and internal, and that the larger and more complex forms augment
the area afforded by the external surface by developing outgrowths
known as gills. These vary greatly in shape, and are situated
in various places. When they are large and complicated, it is
usual to find special arrangements for sheltering them, and also
for maintaining a constant flow of water over their surface.

The higher classes of the Backboned Animals (Vertebrates),
i.e. Mammals, Birds, and Reptiles, live mostly on the land, and
take in ordinary air for breathing purposes, which, after some of
the oxygen has been absorbed from it, is again passed out, heavily
charged with two of the waste products of the body, carbonic acid
gas and water. The essential nature of the process is precisely
the same as in those animals which breathe in water, but these
use the oxygen which is dissolved in the surrounding medium.
An animal that uses ordinary air for breathing purposes, and
which, for brevity's sake, we may call an " air-breathing" animal,
relies in most cases upon part of its internal surface, and back-
boned forms which do this mostly possess lungs, i.e. hollow out-
growths from the under ' side of the throat-region (pharynx) of
the digestive tube, by which the internal breathing surface is aug-
mented. These structures are therefore in a way comparable to
gills, which are also a device for increasing the area over which
respiration can take place.

It is almost if not quite certain that land animals are descended
from aquatic forms which breathed the oxygen dissolved in the
surrounding water, and in the higher backboned animals the proof
of this is unusually clear and particularly striking. If, for example,
we examine an embryo chick, taken, say, from an egg upon which

420

BACKBONED ANIMALS WHICH BREATHE IN AIR 421

the hen has brooded for three days, it will be quite easy to make
out on the side of the neck-region several slit-like openings (vis-
ceral clefts) that communicate with the pharynx, and between
which are thickened bars (visceral arches). These are undoubtedly
equivalent to the gill-clefts and gill-arches of a fish, for they are
situated in exactly the same place, and develop in precisely the
same way, although the clefts are never of use for breathing
purposes, and gill-folds do not grow out from them. In fact they
soon close up, leaving no obvious trace that they ever existed.
(See also p. 381.)

THE ORIGIN OF LUNGS

We have seen that there is considerable ground for the belief
that gill-pouches in fishes and the like have probably been evolved
from pouches on the side of the pharynx which originally per-
formed some other function (see p. 382). And the modification
of old organs into structures having a new use is such a frequent
occurrence that we may well enquire if the lungs of backboned
animals have not been made out of pre-existing structures, as

Fig. 547. — Swim-Bladder of Bichir (Polypterus], reduced and diagrammatic. AP., Aperture on floor of
pharynx leading into swim-bladder.

have the gill-pouches which they supersede. Since Fishes cor-
respond in a broad way to the aquatic ancestors of land-verte-
brates, it is among them we must seek evidence as regards the
evolution of lungs. Many Fishes possess a swim-bladder, con-
taining air and developed as an outgrowth from the front part
of the digestive tube. Although its primary use is to help in
floating and balancing the body, it is known in some instances
to assist the breathing, and it seems a likely enough organ to
undergo modification into a lung. In the Bichir (Polypterus) of
the Nile the swim-bladder is double (fig. 547) and grows out
from the under side of the digestive tube, characters which make
it more lung-like than that of most other forms, in which it
is usually single and grows out of the upper side of the gut.

422 ANIMAL RESPIRATION

Indeed it must not be rashly assumed that the swim-bladder of
one fish is of necessity precisely equivalent to the swim-bladder
of any other form. The best evidence to be obtained in support
of the view indicated as to the origin of lungs is presented by
the remarkable Lung- Fishes (Dipnoi], of which more will sub-
sequently be said. These creatures possess not only efficient
gills, but also a swim-bladder into which ordinary air is taken
for breathing purposes, the purified blood being returned direct
to the heart, just as in the case of an ordinary lung.

Accepting the mode of origin of lungs just outlined, we may
next enquire how these organs become more and more complex
as we follow them up from Lung- Fishes to Amphibia, and thence
to the thorough-going land-groups of Reptiles, Birds, and Fishes.

LUNGS OF AMPHIBIANS (AMPHIBIA)

Taking the Common Frog (Rana temporaries) as a good type,
we find that its lungs are a pair of simple elastic bags, which can
be distended to a considerable size by the taking in of air. The
surface presented by their lining is to some extent increased by
the presence of a number of ridges, which give a honey-comb-like
appearance. The ridges are traversed by delicate capillary blood-
vessels, the blood contained in which is only separated from the
air in the lungs by the thin lining of those organs and the equally
delicate walls of the vessels. Hence diffusion of oxygen into the
blood, and of carbonic acid gas out of it, easily takes place.

We have seen that in the case of gills the maintenance of a
stream of pure water over the breathing surface is provided for
in a number of ways. Renewal of air in a lung is a matter of
equal importance, and the manner in which it is brought about
varies in different groups. As regards the Frog, it is of im-
portance to notice that in this animal the feeding and breathing
tracts are to some extent separated, though the arrangement is
not so perfect as in the cases of the Lancelet, Tunicates, and the
Acorn-headed Worm (see p. 389), where breathing-water and
feeding- water are largely kept apart. It is a general rule among
backboned animals where lungs are present, for the air used in
breathing to pass through the cavities of the organs of smell, and
we see the beginning of this in Lung- Fishes, where these organs
not only possess external nostrils (the only nasal openings present

BACKBONED ANIMALS WHICH BREATHE IN AIR 423

in ordinary fishes), but also internal nostrils, placed just within the
upper lip. It is clearly advantageous for a land-animal to take in
air through the nose, for food is often detected by smell, and this
device increases the chance of finding it. In a Frog the internal
nostrils open rather further back than in a Mud- Fish, and the
external nostrils are valvular. The chief agency by which air
is taken in and passed out is found in the muscular floor of the
mouth, which is moved alternately up and down. The procedure
adopted is somewhat as follows. The mouth being closed, its
floor is lowered, when air passes through the nose into the
mouth-cavity, after which the valvular nostrils are shut. The
floor of the mouth is now raised, and the air is forced into the
lungs. The blood in these organs having been purified to some
extent, the floor of the mouth is again lowered, and the impure air
is drawn out of the lungs into the mouth-cavity. The tip of the
snout is next bent down a little so as to open the external nostrils,
the mouth-floor is raised, and the air forced out through the nose
to the exterior.

The Frog is only partially adapted to a life on land, being
dependent upon a damp surrounding atmosphere, and, taking
advantage of this, it has retained, or perhaps reacquired, the
old method of breathing by means of the external surface of the
body as an accessory to the more specialized respiration effected
by lungs.

In the snake -shaped Csecilians only one lung is properly
developed, for two efficient organs of the kind could not be
packed into the narrow body.

It is interesting to notice, in passing, that some Salamanders
have specialized on quite different and very extraordinary lines
in regard to respiration. Their peculiarity consists in the fact
that they have abandoned lung-breathing altogether, for their
lungs are either entirely absent or else reduced to useless remnants,
while at the same time the structure of the heart has undergone
a corresponding change. How breathing is carried on under such
circumstances is not definitely known, but the skin, the lining of
the mouth and pharynx, and the lining of the intestine have all
been suggested as the parts which supply the place of the absent
lungs. The pretty little spectacled Salamander (Salamandrina
perspicillata) of Northern Italy will serve as an example of these
lungless Amphibians.

424

ANIMAL RESPIRATION

LUNGS OF REPTILES (REPTILIA)

Reptiles (as also Birds and Mammals) have abandoned skin-
breathing, and rely entirely upon their lungs, which in the smaller
members of the class, such as many Lizards, resemble in structure
those of the Frog. But in the larger Lizards, Turtles, Tortoises,
and Crocodiles these organs have become more complex, with the
object of providing a larger breathing surface. Instead of a simple

honey-combing of the internal
surface, complex folds have come
into existence (fig. 548). In this
way the wall of the lung has
become more or less spongy,
with corresponding reduction of
the central cavity. A new method
of renewing the air in the lungs
has also come into existence,
for well-developed ribs are here
present, attached to a breast-
bone below, and jointed on to the
backbone above. By muscular
action the ribs and breastbone

can be swung forwards and downwards, so as to increase the
volume of the front part of the body, by which air is caused to
flow into the lungs. When the muscles concerned cease to con-
tract, these parts move back, mainly as a result of elasticity, to
their original position, and the impure air passes out to the ex-
terior. It may, then, broadly be said that renewal of air in the
lungs depends in Reptiles (as also in Birds and Mammals) on
the mobility of the chest-region (thorax). In cases where the
lungs are complicated and spongy the movements of breathing
cause direct renewal of air in the larger passages only, that in
the smaller ones being purified by diffusion.

In an average Reptile the internal nostrils open further back
than in Amphibians, and thus the feeding and breathing tracts
are rather better separated. Crocodiles exhibit a great advance
upon this condition, for the internal nostrils do not communicate
with the mouth-cavity at all, but with a pharynx (fig. 549), into
the floor of which the top of the windpipe projects (see p. 70),

Fig. 548. — Diagrammatic longitudinal sections through
Lungs, showing increase of breathing surface by in-
growth of folds; 3, main branch of windpipe. A, Newt;
B, Frog or small Lizard ; c, Large Lizard or Tortoise;
D, Turtle or Crocodile.

BACKBONED ANIMALS WHICH BREATHE IN AIR

425

and which is separated by a fold from the mouth-cavity. The
mouth can therefore be kept open under water without fear of

Fig. 549.— Head of Crocodile to show Breathing Arrangements, hinder part in section

BR., Brain; LA., larynx; L.J., lower jaw; N., external nostril; N'., internal nostril; SK., skull. Course of
air entering lungs shown by arrows.

suffocation by entry of water into the lungs. The external
nostrils are valvular, and close when their owner sinks below
the surface. o T

Curious modifications of the breathing-organs
are found in Snakes. As in the snake-like Amphi-
bians (Caecilians), the left lung is reduced to a
mere vestige, the right lung being correspondingly
enlarged. The reason for this arrangement would
appear to be the same in both cases, z>. adapta-
tion to the shape of the long narrow body. An
ordinary Snake feeds on living animals of rela-
tively large size, and, to prevent choking while
the tedious process of swallowing is going on, the
end of the windpipe is drawn out into a project-
ing tube which protrudes from one side of the
mouth.

Snake-like Lizards (Amphisb&no), like true
Snakes, possess but one fully-developed lung, that
of the right side. Fis- sso.-Lungs of

. a Chameleon, showing

The lungs of Chameleons (fig. 550) suggest outgrowths. TR.,wind-

S ^. , V & . ' , pipe (trachea).

the arrangements that Birds possess in a much

more elaborate condition, for the hinder portions of them grow

426

ANIMAL RESPIRATION

WINDPIPE

out into slender thin- walled air-sacs, which penetrate between the
other organs of the body. How far these organs are of use in
breathing is not known.

LUNGS OF BIRDS (AVES)

Although richly supplied with blood, the lungs of Birds are
comparatively small, and instead of being mobile are closely fixed
to the ribs and backbone. From the main air-passages which
traverse them very complex branches are given off, the linings
of which are raised up into folds by which the breathing surface
is greatly increased. A number of large thin-walled air-sacs are

connected with the lungs (fig. 551), and these
not only fill up most of the space between
the other internal organs, but are also, as a
rule, continuous with air-spaces in the bones.
How far purification of the blood takes place
in the neighbourhood of these air-sacs is
doubtful, but in any case it would appear
certain that their presence conduces to the
rapid and frequent flow of air through the
main channels of the lungs, and hence pro-
motes rapid breathing. It has been explained
elsewhere that only Birds and Mammals
among backboned animals are hot-blooded,
maintaining a constant temperature whatever
may be that of their surroundings (see pp.
208, 244). In these two groups the problem
of adapting to lung-breathing requirements a
heart and blood-vessels inherited from gill-
breathing ancestors has been for the first time solved, and
the impure blood poured into the heart from the general body
is kept separate from the pure blood which that organ receives
from the lungs. Hence increased activity, associated with more
efficient breathing, one manifestation of which is a higher body-
temperature. Birds are more active than Mammals, and it is not
therefore surprising that they possess hotter blood (103° to 104° F.
as against about 98° F.).

The relation of air-sacs to flight will be considered in another
section. As to the means by which air is caused to enter and

Fig- 551- — Lungs and Air-sacs
of a Bird, seen from under side.
The air-sacs of only one side are
shown, and two of them have been
cut through and rods pushed into
them.

BACKBONED ANIMALS WHICH BREATHE IN AIR

427

leave the breathing organs, it need only be said that the arrange-
ment is much the same as in Reptiles (see p. 424) when a bird
is standing or walking, but
during flight it would appear
that the breastbone is rela-
tively fixed, while the hinder
part of the backbone is
moved up and down. The
same end is effected in either
case.

Owing to the length of
a bird's neck the windpipe is
correspondingly elongated,
and in some cases (Cranes)
it is thrown into a number
of loops situated in the
breastbone (fig. 552). No
plausible explanation has
been offered as to the use of
this peculiar arrangement,
but it is difficult to believe
that it has not some special
meaning.

Fig- 552- — Windpipe of a Crane

a a a, Windpipe; bb, breastbone; cdef, hyoid bone (which
supports tongue); gg, shoulder-blades; h, right coracoid bone;
/, merry-thought.

LUNGS OF MAMMALS (MAMMALIA) (see vol. i, p. 45-47)

The lungs of a Mammal ^are very complicated spongy organs
which fill most of the thorax (fig. 553). The windpipe divides
into two branches, as a general rule, one for each lung. If one
of these is traced it will be found to branch in a tree-like way,
the smallest, very thin -walled branches being known as bron-
chial tubes. Each of these ends blindly, swelling up into a little
bunch of air-cells, the walls of which are invested by a net-work
of capillary blood-vessels. It is in these air-cells that the blood
is purified, and they present collectively an enormous breathing
surface. In short, the spongy lung of a Mammal is a very perfect
contrivance for packing into a comparatively small space an ex-
tended area over which exchange of gases between the impure
blood and the air can readily take place. Amphibians, Reptiles,

428

ANIMAL RESPIRATION

T).

and Mammals form a series of increasing complexity in this respect,
keeping pace, as it were, with increasing need for rapid oxygenation
of the blood, which is most marked in the highest animals. An-
other series is constituted by the breathing organs of Amphibians,

Reptiles, and Birds, culminat-
ing in the last, which surpass
even Mammals as regards re-
spiratory efficiency. They
have, however, as we have
seen, specialized on rather dif-
ferent lines.

As to the mechanical ar-
rangements by means of which
air is renewed in the lungs of
Mammals, the movement of
ribs and breastbone, so as to
alternately increase and di-
minish the size of the thorax,
takes place in much the same
way as in Reptiles and Birds.
There is, besides, a fresh factor
of great importance in the

Fig. 553. — Air-passages of Lungs of Man

A, Windpipe; B B, bronchi into which this forks; DD, smaller
air-tubes

midriff or diaphragm, a parti-
tion by which the cavity of the thorax is separated from that
of the abdomen. This may be described as a thin, curved
muscle, convex towards the thorax, and possessing a central ten-
dinous part. There are also some strong bundles of muscle,
the " pillars" of the diaphragm, which run from the under side
of the backbone in the region of the loins to the part of the
midriff furthest from the breastbone. The volume of the thorax is
increased in the direction of its length, and the taking in of air
promoted, by the contraction of the muscular margin of the midriff
and of its pillars, so that the edge of this partition becomes flat-
tened. When the muscle ceases to contract, the midriff once more
becomes convex towards the thorax, which therefore diminishes in
volume, as a result of which expulsion of impure air from the
lungs is greatly helped. Indeed the midriff is of the very greatest
importance as a breathing muscle. Backboned air-breathers which
are lower in the scale than Mammals possess an ill-developed
equivalent, though this usually has nothing to do with breathing.

BACKBONED ANIMALS WHICH BREATHE IN AIR

429

As in so many other cases, material already present has been
modified so as to serve a fresh purpose. It is worth while to note
here that the lungs of Mammals, like those of Amphibians and
Reptiles, are very elastic, and are kept continually on the stretch.
Their elasticity greatly aids in the expulsion of impure air from the
breathing passages.

It would be a mistake to suppose that the breathing move-
ments which the walls of the chest execute cause air to rush
into the bronchial tubes and air-cells. It is only the air in the
larger air-passages which is directly renewed in this way, while
purification of the air in the smaller passages and their endings
is brought about by gaseous diffusion.

In Amphibians, Reptiles, and Birds the breathing and feeding
tracts are not very completely separated (except in the case of
Crocodiles), but in Mammals they are
much more perfectly marked off from
one another. The internal nostrils do
not open on the roof of the mouth, but
into the pharynx (fig. 554). It may, in
fact, be said that the two tracts cross
one another in this part of the digestive
tube, on the floor of which is the open-
ing {glottis) which leads into the wind-
pipe. Mammals also present a very
characteristic arrangement by means
of which food is prevented from get-
ting into the breathing organs. For
the front of the glottis is guarded by

1 . n -i /•///• 1*1 Fig- 554- — Mouth, Nose, &c., of Man,

an elastic nap, the epiglottis, which in vertical section

durinO" Swallowing is folded back OVer *» Cavity of nose opening into pharynx

& . CM), and separated from mouth-cavity by

the breathing opening, and constitutes, hard paiate («) and soft paiate w, the latter

,. , . , ending in a rounded projection (the uvula, «),

aS it Were, a SOrt OI bridge OVer below which is the opening between mouth-

which the food passes back into the JS^^^SK^S^sTI^
gullet. Human beings sometimes at-
tempt to speak when this transit of food is in progress, with the
result that the epiglottis springs up and particles make their way
into the windpipe. This is what is popularly called " swallowing
the wrong way ".

It would scarcely repay us to consider in detail the various
modifications which the breathing organs present in the various

43°

ANIMAL RESPIRATION

subdivisions of the Mammals, but the order (Cetacea) which
includes Whales, Porpoises, &c., is of special interest in this
connection, and therefore merits a brief notice. Although these
animals are obliged to come frequently to the surface in order to
breathe or " blow ", they can remain under water without incon-
venience for between three and four minutes. It is not, therefore,
surprising to find that their lungs are of large size, and it may also
be noted that parts of the body-wall are richly supplied with ela-
borate net-works of blood-vessels aggregated into thick masses
(" wonder-nets ") which possibly serve as a means of storing
purified blood, though this is only a conjecture. The midriff is
very thick, and its central tendon comparatively small, and the
unusually large amount of muscle present renders the movements
of breathing particularly vigorous. The most remarkable pecu-
liarity, however, is the very complete separation which takes
place between the breathing and feeding tracts, pretty much
as in a Crocodile (see p. 425), the object being similar, i.e. to
enable the animal to open its mouth under water without risk
of choking from entry of fluid into the air-tubes. The top of
the windpipe is a projecting cone, the end of which fits closely
into the back of the nasal passage, leaving, however, a space on
either side by which food can travel on into the gullet. In order
that the animal may breathe with as small a part of the body out
of water as possible the nostrils are shifted to the top of the head,
and are very close together, or even fused into one, constituting two
or one "blowhole", as the case may be. When in the colder ocean
regions a whale comes to the surface and breathes out air from its
lungs, the watery vapour with which this is abundantly charged is
condensed by the cold so as to become visible. This appearance
has given rise to the common but erroneous idea that a whale
" spouts " out of its blowhole the water that has been taken in at
the mouth. Such a procedure would be impossible for anatomical
reasons, and the superfluous water which is taken in with the
minute animals that mostly constitute the food (see p. 29) really
passes out at the sides of the mouth.

In the newly-born young of Pouched Mammals (Marsupials)
such as Kangaroos, the breathing and feeding tracts are separated
in much the same way as in the Whales and their allies, but for a
rather different reason. Young Marsupials when they first come
into the world are in an exceedingly immature and helpless con-

BACKBONED ANIMALS WHICH BREATHE IN AIR 431

dition. They are placed by the mother in her pouch and affixed
to the long teats there situated. As for some time they cannot
even suck properly, the milk is forced into their mouths by the
contraction of a sheet of muscle which covers the milk-glands,
a procedure which would be pretty sure to cause suffocation were
there not some mechanical device to prevent it.

CHAPTER XXXVII

ANIMAL RESPIRATION— BACKBONELESS ANIMALS
WHICH BREATHE IN AIR

Having considered the backboned air-breathers, we now pass
on to Molluscs, Arthropods, Worms, &c. which live on land or
in fresh water, and use ordinary air in breathing, as contrasted
with members of the same great groups that live in water, salt or
fresh, and respire the air which is dissolved in this.

AIR-BREATHING MOLLUSCS (MOLLUSCA)

The only subdivision of the Molluscs which contains air-
breathing members is the class (Gastropoda) that includes Snails
and Slugs of all kinds. The great majority of these are marine,
but the order of Lung-Snails (and Slugs) has been established for
the reception of most of the species which live on land or, it may
be, in fresh water, and are collectively termed Pulmonates (L.
pulmo, a lung), because they breathe air by means of an organ
which may be called a " lung ", though it is quite different in
origin and nature from the lung of a backboned animal. There
are also certain land-snails (Cyclophorus, Cyclostoma, &c.) which
are not Pulmonates (though they breathe in a similar way), but
belong to the Fore-gilled Gastropods (Prosobranchs).

We have seen (see p. 393) that -in the marine Prosobranchs
there is a cavity opening by a wide slit above the neck, and con-
taining either two (Ormer and Keyhole Limpet) or more usually
but one (Whelk and Periwinkle) plume-like gill. This is the gill-
cavity, and its roof is the mantle, which may be regarded as a sort
of flap that has grown out from the wall of the body. In an
ordinary Limpet (Patella] the gill-cavity has lost both its gills, and
is probably able, by means of its thin roof (which is richly pro-
vided with blood-vessels), to breathe the damp air which surrounds

432

BACKBONELESS ANIMALS WHICH BREATHE IN AIR 433

the animal when it is uncovered by the tide. A Limpet, however,
possesses special gills all round the body (see p. 396), which are
used when the tide is up, and perhaps also breathe air when the
tide is down. It is practically certain that some of the Land-Snails
have sprung from ancestors which lived, like Limpets, between
tide-marks, and were able to breathe both air dissolved in water
and ordinary atmospheric air. Driven by keen competition from
the shore, the descendants of these forms took to living altogether
on dry land, and gave up breathing air dissolved in water, while
at the same time their gill-cavities became specialized as air-
breathing organs. Other Land- Snails probably took origin from
estuarine or freshwater species, in which cases the still earlier
ancestors were doubtless marine in habit.

Examination of a Garden-Snail (Helix aspersa), a common
and very typical Pulmonate, shows the presence of a cavity
opening above the neck, corresponding
precisely to the gill-cavity of a Limpet
both as to position and in regard to the
complete absence of gills. This "lung",
as it may be called for convenience
sake, is of large size, and the inner sur-
face of its thin roof (the mantle) is
raised into a net-work of ridges (fig.
555) by which the surface exposed to
the air is increased in area (compare
p. 424). The muscular floor of the
lung is curved, and by its movements
brings about the passage of air into
and out of the breathing cavity. The
lung does not open in front by a wide Fig 555_Roof of Lung of Land.Snail

Slit, aS did the anCeStral Pill-Cavity tO (Helix), showing inner surface (enlarged).

0 J Impure blood flows to the net-work of ridges

Which It Corresponds, but by a Small and, when purified, is carried by the pul-

, 1 , . • 1 1 i • i monary vein (PUL. VEIN) into the auricle (AU)

rOUnd hole On the right-hand Side. of the heart and thence to the ventricle (VH),

T1_ • c ^1 1 which pumps it to the body; UR., ureter

I his narrowing of the external aper- carryin£ waste from kidney and opening in
ture is in order to prevent the lung *°c"umupper end of figure) close to end of
from drying up, which would prevent

it from performing its function, and by muscular action the com-
paratively small opening can be varied in size or even closed
altogether according to circumstances.

Land- Slugs may be broadly described as flattened-out Snails,

VOL. II.

434 ANIMAL RESPIRATION

and their breathing organs are constructed on the type just
described. The movements of breathing can easily be watched
in a living specimen of the common Black Slug (Arion ater\
which is one of the species in which the shell has disappeared
altogether. The lung is here rather small, and its boundary is
marked by a different texture of skin. As breathing goes on the
pulmonary aperture may be noticed alternately 'enlarging and
narrowing.

Among the commonest air-breathing Snails living in fresh
water are different species of Pond-Snail (Limn&us) and Trumpet-
Snail (Planorbis). These and similar forms are obliged to come
to the surface from time to time for the purpose of breathing.

AIR-BREATHING ARTHROPODS (ARTHROPODA)

The typical air-breathing classes of the Arthropods are: —
(i) the one which contains only the widely-distributed and simply
organized form Peripatus (Prototracheata), (2) Centipedes and
Millipedes (Myriapoda), (3) Scorpions, Spiders, Mites, &c.
(Arachnida), and (4) Insects (Insecta). There are also certain
Crustacea which have taken to a life on land, and have therefore
acquired the power of breathing ordinary air.

PERIPATUS (PROTOTRACHEATA)

This creature possesses in their simplest form the air-tubes
(trachea) which are the most characteristic respiratory organs
of air-breathing Arthropods. Indeed, on this account, these are
often collectively termed Tracheata.

Scattered over the body of Peripatus are a number of small
holes (stigmata), some of which are arranged on either side in
two rows, one above and the other between the stump-like legs.
Each of these holes opens into a small cavity from which arise
a number of delicate air-tubes that penetrate the body to some
extent, the end gained being to supply oxygen to the different
organs, and take away their carbonic acid gas together with
some water vapour. By means of alternate contraction and
relaxation of the muscular wall of the body the air in these
tubes is from time to time renewed. The principle of this
arrangement is quite different from that exemplified by the

BACKBONELESS ANIMALS WHICH BREATHE IN AIR 435

lungs of backboned animals and land-snails. For, as we have
seen, there is marked centralization in the case of these organs,
impure blood flowing to them for purification, after which it is
distributed to the body at large. Air - tubes effect the same
purpose by decentralization, carrying air to and from the different
organs. In Peripatus the breathing organs are not very perfect
or specialized, but they are an early step in the evolution of a
mode of respiration which reaches its highest expression in
insects. We have no certain knowledge of the way in which
air-tubes first took origin, but it is extremely likely that they
are modifications of structures which originally served some
other purpose. Peripatus resembles segmented worms (annelids)
in several respects, and it is pretty certain that the ancestral
forms from which arthropods have been derived were segmented,
more or less worm-like creatures. It is therefore among annelids
that we must look for the organs which by change of function
have become air-tubes. Now the skin in such worms contains
numerous glands, by which various sorts of material are separated
from the blood for various purposes, such, e.g., as the formation
of tubes in which to dwell. And it has been suggested that
air-tubes have arisen from branched skin - glands which gave
up their original function and were specialized for carrying oxygen
to the body.

CENTIPEDES AND MILLIPEDES (MYRIAPODA)

The openings (stigmata] into the system of air-tubes by which
a Millipede or Centipede breathes are placed in two rows, one
along each side of the body. The remote ancestors from which
these forms are descended probably resembled Peripatus in many
respects, and no doubt possessed stigmata scattered over the
body, as well as some with a more regular arrangement (see
p. 434). Later on many of these were done away with, only a
row along each side being retained, as most conveniently situated.
This is another instance of the principle already illustrated by
the cases of gill-slits (see p. 386) and teeth (see p. 14), where
greater efficiency is obtained by reduction in number of structures,
which are in the first instance numerous and unspecialized. We
shall have occasion to see that among Insects the reduction of
these particular organs is carried a great deal further. Each

436

ANIMAL RESPIRATION

of the breathing-pores of a MILLIPEDE opens into a small air-
cavity, from which a bundle of branching air-tubes penetrates
into the adjacent parts of the body in a much more thorough
way than in Peripatus. These air-tubes are also much more
definite in nature than in the last-named animal, and each of
them has a firm elastic lining possessing a spiral thickening,
the object of which is to give flexibility and prevent collapse of
the walls of the tube. Indistinct traces of such a thickening are
seen even in Peripatus. The same end is served in fire-hose
and the like by insertion of a spiral wire into the cavity of a
flexible tube, so that the flow of water may not be interrupted
by any accidental kink. The object in the air-tube is of course
to secure a continuous passage of air.

CENTIPEDES are much more active creatures than Millipedes,
and therefore require more perfect arrangements for purification
of the blood. The breathing-pores open as before along the
sides of the body, where the body wall is much softer than it
is above and below, in which regions the skin is covered by a
strong horny layer. The pores are less numerous and more
specialized than in a Millipede, and the air- tubes which are
connected with them ramify through the body in a more thorough
way. The bunches of air-tubes are also more or less united by

connecting tubes, so as to form a con-
tinuous system, and this considerably
promotes the circulation of air.

The breathing organs of the greatly
specialized Shield-bearing Centipede
(Scutigera)) a very active long-legged
creature, with a short body, differ
greatly from those found in ordinary

Fig. 556.-Breatmng Organs of Shield-bearing Millipedes and Centipedes. There

Centipede (scutigera)

A, TWO of the shields, slightly enlarged, are eight shield-like scales on the

upper side of the body, and at the

siderably enlarged-the blood-space surround- Binder Cttd of Cach of thcSC (cXCCpt

ing the sac is represented in black. V

the last) is a slit which leads into what

may perhaps be called a lung-sac (fig. 556). This is a flattened
bag suspended in a blood-space, and giving rise on either side
to some 300 branched air-tubes, separated only by their thin walls
from the surrounding blood, which it is their office to purify.
The arrangement is specially interesting, because it resembles

BACKBONELESS ANIMALS WHICH BREATHE IN AIR 437

to some extent the breathing apparatus which is characteristic of
Scorpions, of which details will be given later.

It is noteworthy that in animals which breathe by air- tubes
the blood-system is in a comparatively ill-developed condition,
although the other organs of the body may be exceedingly com-
plex and specialized. For while irregular blood-spaces of different
size are found in all parts of the body, there is not, as one might
perhaps expect, an elaborate set of blood-vessels, but only a
heart and, it may be, some exceedingly delicate arteries which
soon merge into the blood-spaces. The heart is often called
the dorsal vessel, being a long thin-walled tube running near the
upper surface of the body. Its sides are provided with numerous
pairs of valvular openings through which blood enters. The
imperfect state of the blood-system is related to the exceptional
nature of the means by which respiration is effected. In, say,
a fish one great use of the heart and vessels is to pump blood
to the breathing organs for purification, but this arduous kind
of work is unnecessary in a centipede or insect, for the air-
tubes of these creatures carry pure air to all parts of the body,
so that the blood of any organ gets rid almost immediately of
any carbonic acid gas which it may have evolved, and at the
same time the corresponding loss of oxygen is made good. The
organs of circulation are therefore relieved of a large amount
of work, and the chief duty which remains to them is that of
carrying nutritive material through the body for repair of waste
and promotion of growth, a duty which can be carried out
sufficiently well without an elaborate system of blood-vessels.

INSECTS (INSECTA) AS AIR-BREATHERS

In the case of typical insects the breathing-pores are com-
paratively few in number, and open into an exceedingly complex
system of air-tubes, which permeate all parts of the body. The
arrangement has already been briefly described for the Cock-
roach (see vol. i, p. 348), in which each of the last two segments
of the thorax and first eight segments of the abdomen bears a
pair of pores — ten pairs in all. The arrangement of some of
the larger air-tubes will be gathered from fig. 557. The internal
organs have a silvery appearance when immersed in water, and
this is caused by the air contained in the minute breathing-

438

ANIMAL RESPIRATION

tubes which ramify on and within them. Looked at under the
microscope these present a very striking appearance. The
breathing-pores are valvular, and provided with minute muscles
by which their size is regulated, it being possible to close them
altogether. The contents of the air-tubes are expelled by the

contraction of muscular bands that stretch
from the upper to the lower side of the body,
which is therefore flattened when they come
into action. On these muscles ceasing to
contract, the body resumes its former shape
as a result of elasticity, and air is conse-
quently drawn into the breathing-tubes. But
if the breathing-pores kept fully open all the
time air would only be renewed in the larger
air -tubes, while the minute branches would
have to rely upon diffusion for the purification
of their air. This, however, is not the case,
for competent authorities state that the breath-
ing-pores are closed for part of the time
the breathing-muscles are acting, so that air
is forced into many of the small air-tubes,
the contents of which are thus directly re-
newed. This is strikingly in contrast to

Fig. 557 -Dissection of Cock-

roach (Periplaneta orientalis , i -i '.11 r 1 i •

from above, to show chief air-tubes what happens in the lung of a human being

(enlarged); r-

segments of abdo-

Qr oter ger vertebrate, wllCFC the Smallest

air-spaces are of such a delicate nature that

they would be liable to injury if currents of air constantly passed in
and out of them, and which therefore get rid of their carbonic acid
gas and keep up their supply of oxygen by gaseous diffusion. As
even the minutest branches of the insect's air-tubes are furnished
with a firm elastic lining they are not so liable to injury by a
tidal movement of air, such as must take place when air is
forced into them, and later on squeezed out again. The view
is here taken that air-tubes serve for getting rid of carbonic
acid gas as well as for the introduction of fresh oxygen, but
some zoologists maintain that only the latter purpose is served
by them. It must, indeed, be confessed that our knowledge of
the breathing of insects is very imperfect, and in this, as in so
many other directions, there is abundant scope for research upon
the physiology of lower forms.

BACKBONELESS ANIMALS WHICH BREATHE IN AIR 439

- r.g.

There is a very interesting kind of modification observable in
the breathing organs of Insects, such as Bees and Locusts, which
possess the power of rapid flight, for in such cases the air-tubes
swell into a varying number of air-
sacs (fig. 558), the result being to
give the body a larger bulk in pro-
portion to its weight than would
otherwise be the case. This is
curiously reminiscent of the state
of things in Birds, where the lungs
communicate with a system of large
air-sacs, and in both kinds of ani-
mal the use of the arrangement is
to be sought in connection with the
power of flight, which will be dis-
cussed in a later section. But it
must not be supposed that the air-
sacs are equivalent in the two cases, rig. 558. -Dissection of Honey-Bee (Apis meiufica},
for the lungs and air-sacs of Birds ,,, Two of the breatHnlpores (stigmata),^,, .air-
arise by outgrowth from the diges- ^^^^™*^™^ £

tive tube, While the air- tubeS and stomach ;«., excretory tubes ;r, rectum; r.^., rectal
r T l ' 1 • • 1 glands.

air-sacs of Insects develop in the

first instance as simple in-pushings of the skin, which gradually
become more and more complicated until the adult condition
is reached.

AIR-BREATHING AQUATIC INSECTS. — Some Insects live in
water during part or all of their existence, in spite of which
they may be dependent upon ordinary air for breathing pur-
poses. Special devices are then generally present, having re-
lation to the aquatic habit. The Great Water- Beetle (Dytiscus\
for example, lives entirely in the water during the larval part
of its life, and mostly so when adult. The elongated predaceous
larva has but one properly developed pair of breathing -pores,
placed at the tip of the tail, which is from time to time pro-
truded from the water to allow air to enter the respiratory tubes.
This is facilitated by the presence of numerous hairs at the hinder
end of the body, causing this region to float up more readily than
the head end. The adult Beetle is furnished with a full comple-
ment of breathing-pores, which open above into a space of which
the floor is formed by the upper side of the body, and the roof by

440 ANIMAL RESPIRATION

the wing-cases. This space serves as a reservoir for air, which
the beetle carries about with it under water, and renews from time
to time by coming to the surface and pushing out the hinder end
of its body. At the same time air is taken into the last pair of
breathing-pores, which are much enlarged and equivalent to those
present in the larva.

The little Whirligig Beetles (Gyrinus), which may often be
seen moving about in an erratic manner on the surface of ponds
and streams, are in the habit of diving when alarmed, carrying
down a bubble of air with them which enables breathing to go
on during submersion, and prevents water from getting into the
breathing-pores. The Great Black Water- Beetle (Hydrophilus
piceus) lives under water, and carries air about in a manner
different from that described for Dytiscus. A large part of the
body is covered with closely-set down, in which air is entangled,
so that the breathing-pores are kept dry and are able to carry on
their work. This air, of course, requires renewal from time to time,
and there is a remarkable kind of adaptation by which this can be
managed without the Beetle having to leave the water. For the
ends of the feelers (antennae) are broad and hairy so as to fit them
for acting as ladles, by means of which air-bubbles are dragged
under water and applied to the downy surface of the body, to
which they adhere.

Interesting breathing arrangements are found in some of the
Water- Bugs, which are either purely aquatic or live chiefly on
the surface. Pond- Skaters (Hydrometridae) are of the latter kind,
and almost everyone must have seen them moving swiftly about
in the way that has suggested their name. Like the Whirligig
Beetles, they are able to dive, at which time they are completely
surrounded by a film of air, which sticks closely to the velvety
surface of their bodies. It is, however, easy to drown them.
The Water- Boatmen (Notonectid&)> which swim actively about
on their backs, are also provided with an arrangement of hairs,
enabling them to carry a supply of air under water.

The ravenous Water -Scorpions (Nepidce) are thoroughly
aquatic in habit. Both in the broad flat kind (Nepa) and the
lean hungry -looking one (Ranatra) the only breathing -pores
present are placed at the tip of the tail, from which projects a
long narrow tube composed of two closely-interlocking halves.
It is supposed that the tip of this organ is protruded from time

BACKBONELESS ANIMALS WHICH BREATHE IN AIR 441

to time at the surface for the purpose of taking in air, but the
details have not yet been made out.

Fig. 559. — The Drone- Fly {Eristalis tenax] and its larva, the Rat-tailed Maggot

The larvae of a great many Two- winged Insects (Diptera) live
in water, and in many cases come to the surface to breathe air.
It is then usual to find a breathing- tube at or near one end of the

44 2

ANIMAL RESPIRATION

body, with which are connected the only breathing-pores devel-
oped. For instance, the larva of the Drone- Fly (Eristalis tenax)
is commonly known as the "rat-tailed maggot", on account of
its possessing such a tube at the hinder end of the body (fig. 559).
Special provision for a supply of air is particularly necessary in
a form like this, for it lives in liquid filth, and so can only be called
" aquatic " by courtesy.

The larvae of the Common Gnat (Culex pipiens) are little red
wriggling creatures, abundant in stagnant water. Each of them

has a breathing-tube near the end of
its tail, and every now and then comes
to the surface for air, at which times
the tip of the tube is pushed above
the surface, and its valvular aperture
opened. The valve is again closed
when the larva has taken in sufficient
air for the time being. Later on the
quiescent pupa-stage in the life-history
is reached, during which the creature
floats at the top of the water, breath-
Fig. 56o.-Larva (on left) and Pupa (on right) mg nQW by ttlCanS Of a COUple of tubeS
of Common Gnat (Culex pipiens], enlarged °. * •*•

which project from the first ring of the

thorax, the larval tube having disappeared. The pupa, however,
is not so completely passive as the corresponding stage in a beetle
or butterfly, for it is able to dive if necessary to escape any danger
which threatens (fig. 560).

SCORPIONS, SPIDERS, AND MITES (ARACHNIDA)
AS AIR-BREATHERS

Upon the under side of the broad part of a Scorpion's abdomen
four pairs of oblique slits are to be seen, opening into complex
lung-books, which have been compared to plate-like appendages
found in the corresponding region of a King- Crab. The air-
containing cavity of each of these structures is largely filled by
a considerable number of thin plates, which project inwards almost
like the leaves of a book. The plates contain numerous blood-
spaces, and collectively present a very large breathing-surface. It
will be seen that the breathing organs are much more localized
than those of the air-breathing arthropods already described, and

BACKBONELESS ANIMALS WHICH BREATHE IN AIR 443

this necessitates a more complex blood-system, with better defined
and more numerous vessels.

Whip-Scorpions possess two pairs of abdominal lung-books,
similar to those of Scorpions. This is also the case with the larger
Spiders, such as the bird -catching form
(Mygale avicularia) of South America
(fig. 561). The smaller species, such as
the familiar Garden- and House-Spiders,
present a modification of this arrangement.
They retain the front pair of lung-books,
but the hinder pair are replaced by two
sets of air-tubes, resembling in character
those of insects, though possibly of dif-
ferent origin.

The remaining groups of Arachnids
possess air-tubes in those cases where
special breathing organs are present at all.
This is the case, for example, with many
of the small forms known as Mites, of
which Red "Spider" ( Tetranychus telarius]
is so provided. The Cheese- Mite (Tyroi-

. x . • -11 • ' i

glyphus szro) is an instance illustrating the

/- • 1 1 1 .

aDSenCe OI Special breathing Organs. In

, rr , , ,

such cases respiration is effected by the

general surface of the body, as in many small animals of widely

different kind.

AIR-BREATHING CRUSTACEANS (CRUSTACEA)

We have seen that the lung-chamber of an ordinary land-snail
(see p. 433) is really to be looked upon as a gill-cavity from which
the gills have disappeared, and which has acquired a new way of
breathing. Very much the same sort of statement can be made
as regards the thorough-going Land-Crabs, creatures, no doubt,
descended from ancestors which lived between tide -marks, and
gradually came to depend less and less upon the air dissolved in
sea- water. In such a land-crab the large gill-chamber has been
converted into a lung, numerous folds and outgrowths being pre-
sent on its lining, which offer a large surface for exposure to air.
At the same time the gills have been very much reduced in size,
and may even have practically disappeared.

Fig. 561. — Mygale (partly dissected)
from below

mg-legs, cut short ; L.B., lung-books, one

of which is cutopen to show leaflets :ST.,

openings into lung-books; M, a muscle-
band; N., nervous system; ov., ovary;
SP., spinneret.

444 ANIMAL RESPIRATION

The familiar land crustaceans known as Wood- Lice, common
under stones and in the crevices of walls, present another instance
of the same sort of thing. We here find that the limbs of the
abdomen are used in breathing, and each of them consists in many
cases of an outer and inner plate. The latter is delicate in texture,
and does the respiratory work, while the outer plates are protec-
tive, and also prevent desiccation. The latter point is one of
some importance, for it is probable that all sorts of wood-lice
require damp air for breathing purposes. We have, in fact, a
condition which is half-way between ordinary air-breathing and
the state of things characteristic of thorough-going aquatic forms,
which use the air dissolved in the water by which they are sur-
rounded. It may also be added that in some kinds of Wood- Lice
the outer plates of the abdominal limbs are hollowed out by
air-cavities, especially in certain cases where the inner plates have
altogether disappeared.

LAND NEMERTINES (NEMERTEA), EARTHWORMS AND
LAND LEECHES (ANNELIDA), AND LAND PLANA-
RIANS (TURBELLARIA) AS AIR-BREATHERS

None of the forms mentioned in the above heading possess
special breathing organs, but respire by means of the skin, which
is unable to perform its work unless kept moist. This is effected
for the most part by means of a slimy substance secreted by the
epidermis, just as in Frogs (see p. 423), which breathe partly in
this way. It also follows that these various terrestrial forms can
only live in damp places, being even more dependent upon a
humid atmosphere than Wood- Lice (see above).

LAND NEMERTINES have been observed in widely separated
parts of the world. One very interesting form (Geonemertes
Palaensis) was discovered by Semper in so remote a locality as
the Pelew Islands of the Pacific, where it lives among fallen
leaves and the roots of trees.

EARTHWORMS. — When animals which possess a distinct blood-
system breathe by means of the skin, that organ is always richly
supplied with blood, an arrangement which is obviously favourable
to absorption of oxygen and elimination of carbonic acid gas from
the system. The Earthworms furnish a case in point, beside
which the skin is not only of slimy nature, but there is also a

BACKBONELESS ANIMALS WHICH BREATHE IN AIR 445

remarkable arrangement for keeping it moist and free from germs.
The various organs of such worms are situated within a spacious
cavity (body-cavity) containing a clear lymph-like fluid, and com-
municating with the exterior by means of a row of valvular pores
placed on the middle line on the upper surface. Through these
the fluid can be forced out from time to time, serving not only as
a lubricant, but also, it would seem, as an antiseptic, checking the
development of the numerous germs present in the surrounding
earth, some of which might otherwise grow into injurious moulds,
&c., upon the surface of the body.

LAND LEECHES, like almost all their aquatic relatives, breathe
solely by means of the skin, the blood-supply of which penetrates
even into the epidermis, a most unusual arrangement, but one
which brings the blood very close to the surface, and promotes
its rapid purification. This is probably correlated with the un-
pleasantly active habits of these creatures, regarding which
Semper (in Animal Lije] speaks as follows: — "We know . . .
that there are a tolerably large number oi true aquatic animals
which constantly or occasionally live on land. To these, for
instance, belong the true land leeches, as they are called, which
live in the forests of India and the Indian islands, sometimes in
such enormous numbers that it is quite impossible for men to exist
in them even for a few hours. I myself have often been driven
out of the woods of Luzon and Mindanao [in the Philippines],
which are very favourable spots for insects and land-shells, by
the myriads of leeches living on the trees and shrubs, from which
they fall like a drop of dew on any human passer-by ; and I once
read that a whole English battalion had to beat a retreat during
the Sikh rebellion because they were attacked in a wood by these
small blood-suckers in such numbers that passing through the
wood was not to be thought of. They dry up with particular
facility; but as the air in these forests is constantly saturated with
moisture, even in the driest season, they live in India in the open
air on trees quite as well as their nearest allies, the medicinal
leeches, do here in Europe in the water." This quotation also
illustrates very well the easy transition between breathing in water
and breathing in damp air which some groups of animals exhibit.

LAND PLANARIANS are members of a comparatively lowly group
of Flat- Worms (Turbellaria) in which the body is not made up
of rings or segments as in a Leech or Earthworm, and is there-

446 ANIMAL RESPIRATION

fore said to be unsegmented. The skin is also clothed with
cilia, by means of which a gliding sort of locomotion is effected.
Breathing is in all cases effected through the skin, and in the
aquatic Planarians (which constitute the large majority of the
group) the air dissolved in water is utilized for the purpose,
while in the land-forms damp air is the breathing medium. We
have in fact exactly the same sort of transition from one kind
of breathing to the other as in the case of leeches. Land
Planarians are found all over the world, especially in the tropical
regions, where they attain a considerable size (as much as 18
inches in length) and are often brilliantly coloured. One such
large form (Bipalium Kewense) has been accidentally imported
in the earth surrounding the roots of tropical plants into Kew
Gardens and many other places. In reference to the compara-
tively small and inconspicuous European species, Gamble remarks
(in The Cambridge Natural History]'. — "In Europe there are
only two or three indigenous Land Planarians, of which Rhyncho
desmus terrestris is the most widely distributed, and has been
found in moist situations for the most part wherever it has been
looked for. It measures about % inch in length, and is dark
gray above, whitish below, and bears a pair of eyes near the
anterior extremity." In regions which have alternate wet and
dry seasons the Land Planarians tide over the latter, and protect
themselves from being dried up, by burrowing into the ground
and surrounding themselves with a sort of case made of hardened
slime. A similar protective device has been noticed in regard
to Earthworms, which during a dry summer may sometimes be
found twined together in a sort of ball deep down in the ground.

CHAPTER XXXVIII
ANIMAL RESPIRATION— AMPHIBIOUS VERTEBRATES

The term "amphibious" is often applied to creatures like
the Hippopotamus, which, though land-animals as regards struc-
ture, spend a large part of their time in the water. But, scien-
tifically speaking, this is on a par with the well-known definition
of an Amphibian as "an animal which cannot live on land and
dies in the water ", which utterance is supposed to have been
derived from a perennial source of oracular knowledge, i.e. the
answers to examination papers. Zoologists apply the term
amphibious to animals which throughout life are able to breathe
both air dissolved in water and ordinary air, or which carry on
the former mode of respiration during the earlier part of their
existence, becoming air-breathers in the limited sense when
adult.

So far as backboned animals are concerned, it is convenient
to consider Amphibious Fishes in the first place, and then to
deal with Amphibians proper, such as newts, frogs, and their
kindred.

AMPHIBIOUS FISHES (PISCES)

There can be no doubt that land-vertebrates are descended
from aquatic ones, and since these last are typically represented
at the present day by fishes, it is among such animals that we
must look for transitional forms which help to bridge the gap
between the inhabitants of the water and the dwellers on land.

Among ORDINARY BONY FISHES (Teleostei) there are a number
of species which, though not properly speaking amphibious, are
very tenacious of life, and can exist for some time on land, if
surrounded by a damp atmosphere. It is among freshwater
fishes that this kind of peculiarity is best marked, and the Com-
mon Eel (Anguilla vulgaris) is the most familiar instance. This

447

448 ANIMAL RESPIRATION

creature is able to make its way through damp herbage for con-
siderable distances, the object apparently being migration from
one pond or river to another. In correlation with this habit the
external breathing aperture is narrowed somewhat as in a mud-
skipper (see below), though most likely this character was first
acquired to hinder mud from making its way into the gill-cavity
and blocking up the gills.

Carp and their allies are often extremely tenacious of life,
some of them being able to thrive in stagnant water, and others
to endure exposure to the air for a considerable time, provided
it be not too dry. Regarding the Common Carp (Cyprinus
carpio], Giinther remarks (in The Stiidy of Fishes]'. — " They
can also be preserved alive for a considerable length of time out
of the water, especially if care be taken to moisten them occa-
sionally as they become dry. Advantage is often taken of this
circumstance to transport them alive, by packing them among
damp herbage or damp linen; and the operation is said to be
unattended with any risk to the animal, especially if the pre-
caution be taken to put a piece of bread in its mouth steeped in
brandy!" The Tench (Tinea vulgaris) can live in stagnant water
of so foul a nature as to be fatal to most other fishes, which
suggests that it probably makes up for the deficiency of oxygen
by rising to the surface to take in air.

Cases like those just described lead on to others where the
term amphibious is correctly applied. The most striking marine
forms of this kind are the little Mud-Skippers (Periophthalmus],
which hunt for small crustaceans and the like between tide-marks
on the shores of the I ndo- Pacific (see p. 87). To prevent the
gills from drying up at such times the external opening behind
the gill-cover is not a wide slit as in, say, a herring or perch,
but has been reduced to a comparatively small hole. It also
appears that the gill-cavity is relatively spacious, and most pro-
bably its lining performs the functions of a lung. Another very
interesting fact concerning the breathing of these fishes is thus
described by Hickson (in A Naturalist in Celebes]'. — " Their
position is usually one of clinging to the edge of the rocks or
mangrove roots by their fins, with their tails only in the water.
. . . The fact that they live the greater part of their lives with
their head and gills out of water suggested to me an investigation
of their respiratory organs, as I thought it possible that they

Fig. 562. — i, Roach (Leuciscus rutilus] and 2, Bleak (Alburnus lucidus)
VOL. II. 449

450 ANIMAL RESPIRATION

might possess some interesting modifications of the swim-bladder
to enable them to breathe the air. It was not, however, until . . .
1887 that an explanation of the mystery of their respiration
occurred to me — namely, that the respiration is mainly performed
by the tail. Since then Professor H addon has been carrying on
some experiments in Torres Straits, and has shown that this ex-
planation is correct. It seems at first sight a very extraordinary
thing that a fish should have become so modified by change of
habit as actually to have transferred the chief part of its respira-
tory functions from its gills to its tail. It is a well-known and
generally recognized fact, however, that in all the Amphibia the
skin plays a very important part as an organ of respiration, and
it is quite possible that the thin skin between the fin -rays of many
fishes also acts as an accessory to the gills and performs the same
function. If this is proved to be the case we should have to look
upon the tail of Periophthalmus as an example of an organ dis-
charging a function which is performed in a lesser degree by the
tails of many if not of all fishes."

Among freshwater forms some of the carp -like fishes, such
as the Bleak (Alburnus lucidus) and Roach \Leuciscus rutilus]
(fig. 562), are definitely known to rise from time to time to the
surface in order to swallow air, since the oxygen dissolved in the
surrounding water is not sufficient for their breathing processes.
It therefore follows that such forms, extraordinary though the
statement may seem, can be drowned by keeping them below
the surface. Definite experiments have been made which prove
this. For example, Semper (in Animal Life] makes the follow-
ing statement: — "If we prevent the species of Leuciscus [i.e.
Roaches, &c.] from coming to the surface of an aquarium by
placing a wire net just below the surface of the water, so that
they cannot gulp the air, they soon die, even when an ample
supply of highly aerated water is constantly added ". It is sug-
gested that part of this air is breathed by the lining of the
digestive tube, and this is definitely known to be the case in the
small freshwater fishes called Loaches, which also are in the
habit of swallowing air. Two species of Loach are native to
Britain, the Common Loach (Nemachilus barbatulus) and the
Spiny Loach (Cobitis tencza). But all these freshwater forms
quickly die when taken out of the water.

It is, however, by tropical freshwater fishes that a double

AMPHIBIOUS VERTEBRATES 451

mode of breathing is most frequently exemplified. This is partly
related to the fact that in the dry season the smaller streams of
such regions are liable to great reduction in size, and the habit
is also favoured by the great moistness of the air during the
wet season. The Snake-headed Fish (Ophiocephalus] of India,
for instance (fig. 563), possesses what is generally termed an

Fig. 563. — Indian Snake-headed Fish (Ophiocephalus)

accessory gill-cavity covered by the upper part of the gill-cover,
but this cavity does not contain any gill-folds and is probably
to be looked upon as constituting a sort of lung. The habits
of these forms are described by Glinther (in The Study of Fishes)
in the following words: — " Like other tropical freshwater fishes,
they are able to survive drought, living in semi-fluid mud, or
lying in a torpid state below the hard-baked crusts of the bottom
of a tank from which every drop of water has disappeared.
Respiration is probably entirely suspended during the state of
torpidity, but whilst the mud is still soft enough to allow them
to come to the surface, they rise at intervals to take in a quantity
of air, by means of which their blood is oxygenized. This habit
has been observed in some species to continue also to the period
of the year in which the fish lives in normal water, and individuals
which are kept in a basin and prevented from coming to the
surface and renewing the air for respiratory purposes are suffo-
cated." The same kind of specialization is carried still further
in the Climbing Perch (Anabas scandens], where the cavity above
the gills has its lining raised into a number of folds, by which
the breathing surface is largely increased (fig. 564). This fish
is known to come out of the water and undertake comparatively

452

ANIMAL RESPIRATION

lengthy excursions on land, while it is even accredited with the
power of climbing trees, and has been named in accordance with
this belief.

The arrangements described for the last two sorts of fish,
though extremely interesting, do not form a stage in the evolu-
tion of the backboned animals which live on land. And we
now proceed to the consideration of those fishes which do throw

Fig. 564. — Climbing Perch (Anabas
scandens}. Side of head dissected to
show accessory breathing organs.

iff

Fig. 565. — Dissection of a Bitterling (Rhodeus amarus)

bl, Swim-bladder narrowed in the middle; s, gullet; d, intestine;
// liver; v and h, auricle and ventricle of heart; «, kidney; e, ovary;
Ig, egg-laying tube (rarely present in fishes).

some light on the way in which such forms have originated
from aquatic ancestors.

EVOLUTION OF THE LUNGS OF BACKBONED ANIMALS. — It has
already been pointed out (see p. 421) that the lungs of a Frog,
Lizard, or other air-breathing land- vertebrate are comparable to
the swim-bladder possessed by many fishes, serving in them to buoy
up the body and helping to maintain its equilibrium (fig. 565).
The swim-bladder is developed as an outgrowth from the front
part of the digestive tube, with which it often remains connected
throughout life by means of a pneumatic duct. It contains a
mixture of nitrogen and oxygen, the latter gas being more
abundantly present in freshwater species. In some of the Ordi-
nary Bony Fishes (Teleosts) it has been shown that air is actually
taken into this organ from the exterior, and that this furnishes
what may be called a new method of breathing, supplemental to
that of the gills. Indeed it has been proved in some such cases
that the gills alone cannot do all the necessary work of respiration.
For instance, it has been found that in some of the freshwater
fishes of tropical South America (species of Erythrinus] suffocation
quickly takes place if air be prevented from entering the swim-
bladder.

AMPHIBIOUS VERTEBRATES 453

Among recent Ganoid Fishes the Gar- Pike (Lepidosteus] and
Bowfin (Amid), which live in the fresh waters of North America,
both possess a swim-bladder which is better adapted to act as
a lung than is the case in forms so far considered, since its lining
is raised into complex folds which give a large surface through
which exchange of gases can go on. The Bowfin has been
observed to come from time to time to the surface for the purpose
of swallowing air. The swim-bladders so far described are un-
paired structures which grow out from the side or upper surface
of the gullet, facts which have been cited as opposed to the
view that lungs have evolved from organs of the sort, for lungs
are paired and grow from the under side of the gullet. The
first point is of little importance, for the lungs are represented
by an unpaired outgrowth when they first appear, this later on
dividing into two branches. It may be noted here that both
in the Gar- Pike and Bowfin the air-bladder is narrow in the
middle and broad at the sides, giving a sort of incipient double-
ness. Neither objection applies when we come to examine the
air-bladders of the archaic African ganoids, the Bichir (Polypterus)
and Reed- Fish (Calamoichthys), for in both of them the swim-
bladder is double, and grows out from the under side of the
gullet. The lining of this organ being smooth in these fishes,
the breathing work done by it is probably less in amount than
in the two ganoids first mentioned. It is worth while noting
that Bichir and Reed-Fish belong to a distinct and much more
ancient group than other living forms to which the term ganoid
is commonly applied. Fishes of similar kind were exceedingly
abundant in some of the older geological periods. It is hence
extremely probable that in the first instance the swim-bladder of
fish-like forms grew out from the under side of the gullet, but
in most cases its point of origin has gradually shifted round to
the upper side as a matter of convenience, since this organ is
usually placed immediately below the backbone (and above the
digestive tube), a position most suitable for the proper performance
of its duties in regard to the maintenance of equilibrium.

Lung- Fishes (Dipnoi). — The contention that lungs have
evolved from swim-bladders receives its strongest support from
the structure and habits of the remarkable Lung-Fishes, of which
the three existing forms, native to the rivers of Australia, Africa,
and South America, are the last survivors of an exceedingly an-

454 ANIMAL RESPIRATION

cient and once extensive group. They are admittedly amphibious,
and have received both their popular and scientific names (Gk.
dis, double; pnoe, breath) in acknowledgment of the fact. So
much do they differ from other fishes in structure that some zoolo-
gists place them in a separate class, and, since in many respects
they resemble newts, salamanders, and the like, they may broadly
be considered as a sort of half-way stage between Fishes proper
and Amphibians.

The Australian Lung- Fish (Ceratodus] is the least specialized
of the three living representatives of its group, and its lung-like
swim-bladder is unpaired (but divided by furrows into right and
left parts), though it grows out from the under side of the gullet.
The lining of this organ is raised into folds, and it returns purified
blood to the heart, which we find in consequence to be partly
divided into right and left divisions by a partition, so as to keep
apart in some degree the two kinds of blood which are poured
into it. It is, in fact, the first crude attempt at solving the pro-
blem of converting a fish-like type of heart, which receives only
impure blood and pumps it to gills for purification, into the kind of
heart found in typical air-breathing Vertebrates, which receives both
pure and impure blood, that are only disposed of to the best advan-
tage when kept separate (see vol. i, pp. 242-244). It is only the
two highest classes of backboned animals, i.e. Birds and Mammals,
that have attained to a full solution of the problem, and have
succeeded in keeping the two sorts of blood completely separate.
That the members of these two classes are hot-blooded is one
outcome of this feat, and to it they owe, in no small degree,
their present position as the dominant inhabitants of the land.
For long ages Reptiles were the leading terrestrial forms, but
never completely succeeding in converting circulatory organs in-
herited from aquatic ancestors into structures thoroughly adapted
to air-breathing, ultimately had to give place to Birds and Mam-
mals, in which the course of evolution led to more satisfactory
results in this and certain other respects.

Returning from this digression to Ceratodus, it is to be
noted that this creature is only found in a remarkably restricted
area, being limited, in fact, to two small Queensland rivers, the
Burnett and Mary, each of which is practically a chain of deep
water-holes, connected by comparatively shallow reaches. Fossil
evidence proves that the area of distribution was once very much

AMPHIBIOUS VERTEBRATES 455

more extensive. There is abundant testimony showing that this
fish actually does breathe air. For example, Semon, who has
recently studied the habits of Ceratodus in its native rivers, writes
as follows on this point (in In the Australian Busk): — "As afore-
said, Ceratodus is a representative of the almost exterminated
class of Dipnoi or lung-fish; that is to say, fish possessing gills
by which they breathe like other fish, but also an air-bladder,
the construction and function of which very much resembles that
of a lung. What does Ceratodus use this lung for, since it does
not go on land, and therefore is not forced to adapt itself to extra-
aquatic conditions of breathing and living. . . .? That the fish
uses its lung for breathing I noticed hundreds of times. Near the
river area it haunts one occasionally hears a dull groaning sound.
This is produced by the fish, which comes up to the surface at
certain intervals to empty the breath from its air-bladder and
to take in fresh air. I readily proved Ceratodus to be the author
of this strange noise when later on I kept the fish alive in great
barrels and self-dug water-holes. I then saw them appear at the
surface every thirty or forty minutes and lift the tip of their snout
above the water, at the same time uttering the afore-mentioned
grunting noise. Still I was unable to make out whether it is
produced by the expiration of the foul air or the inspiration of the
fresh, and how or where it originates." The author then goes on
to explain how the possession of a lung is related to the actual
conditions of life: " At the same time, like any other fish, Cera-
todus makes use of its gills, and is by no means able to exist
on land. If taken out of the water and prevented from getting
back, its gills soon dry up and the animal dies. Nevertheless
its lungs are of great importance to the fish during the dry season,
for when the water evaporates over a wide area and the river gets
reduced to some few water-holes, the dimensions of which natur-
ally decrease from day to day, an immense accumulation of river
inmates takes place within these last havens of refuge. The
water thus rapidly becomes foul and putrid by rotting animal and
vegetable substance, and the fishes die in numbers. Mr. W. B.
Maltby of Gayndah told me that he had once emptied a big but
not very deep water-hole, which was approaching dryness. The
little water at its bottom was filled with dead mullets, perches,
and other fishes, and the whole was putrid with fish corpses.
Some Ceratodus, however, which were contained in this pool were

456 ANIMAL RESPIRATION

perfectly lively and at their ease, and not in the least disturbed by
finding themselves among these most unsanitary surroundings.
This is the occasion when Ceratodus enjoys the advantage of
its lungs. Not on land, not during a summer sleep in the mire
or in a cocoon are they most serviceable, but in an extremity of
this kind, when they furnish the only means by which the fish
manages to outlive the most unfavourable conditions of its native
rivers."

The African Lung- Fish (Protopterus), specially abundant in the
basins of the White Nile and Congo, is more specialized than its
Australian congener, and its swim-bladder is modified into a
double lung with spongy wall. It is commonly found living in
swamps or shallow streams which practically disappear during the
dry season, and spends that part of the year in a torpid condition
(summer sleep), imbedded in the mud, and surrounded by a sort
of capsule or cocoon formed by the hardening of slime secreted
by its skin. In this state, it is said, the natives dig it out for
food, and dormant specimens of the kind have been transported
to Europe, surrounded by the capsule and a mass of hardened
clay. When placed in warm water these investments readily
break down, and the fishes thus released are none the worse for
their long journey. The newly-hatched Mud-Fish possesses not
only internal gills, but also long, plume-like external gills (like
those of a tadpole), which persist in the adult, though in a reduced
state.

The American Lung- Fish (Lepidosiren), native to the basins
of the Amazons and Paraguay Rivers, closely resembles the Afri-
can form in the structure of its breathing organs and in its habits.
Prof. Graham Kerr has shown that, during the egg-laying season,
numerous scarlet filaments grow out from the pelvic fins of the
male in a feather-like way, and he considers these to be acces-
sory breathing organs.

AMPHIBIANS (AMPHIBIA)

Certain Tailed Amphibians (Urodela) are amphibious in the
adult condition, possessing a pair of simple lungs, and also gills.
This is the case with some of the Salamanders, creatures which
may be regarded as first cousins of the Newts or Efts which
abound in many of our ponds and ditches. In the Hell- Bender

AMPHIBIOUS VERTEBRATES 457

(Cryptobranchus later alis] of the Mississippi, the gills are said to
be " internal ", being in the form of folds on the walls of gill-
pouches, much like those of many fishes, and a gill-opening is
present on the left side of the neck. These gills are evidently on
the down-grade, and are still further reduced in the Giant Sala-
mander (Megalobranchus maximus) of Japan and China, where the
gill-pouches are fewer in number and there is no external gill-
aperture.

A few adult Amphibia supplement their lungs by tufted ex-
ternal gills growing out from the sides of the neck. One of
these forms is the Olm (Proteus anguineus), found in the under-
ground waters of Carniola. In shape it resembles an eel, but
possesses small fore- and hind-legs. A somewhat similar form,
the Siren Salamander (Siren lacertina), inhabits the pools of
swampy districts in the south-east of the United States. In this
species the hind-legs have disappeared altogether.

The large majority of adult Amphibians, such as ordinary
Salamanders, Newts, Toads, and Frogs, breathe only by means
of lungs (and skin) in the adult condition. Like all other Am-
phibia, however, they begin life as fish-like tadpoles, which possess
gills, and are at first devoid of limbs. If they did not develop
beyond this stage they would undoubtedly be classified as fishes.
This remarkable life-history is the most remarkable characteristic
of Amphibians, to which, indeed, their name has reference (Gk.
amphi, both; bios, life). Just as the Lung-Fishes bridge the gap
between ordinary Fishes and Amphibia, and demonstrate how
lungs have been evolved from swim-bladders, so do the Amphi-
bians lead up to the thorough-going land-groups of Reptiles,
Birds, and Mammals, which at no period of life possess gills,
although in the early stages of their existence gill-arches and
gill-clefts remain as a testimony to their aquatic ancestry.

If we follow the development of a common Grass Frog (Rana
temporaria) we shall find that the young tadpole breathes by three
pairs of plume-like external gills, much like those present in the
adult Olm and Siren. Later on these are replaced by "internal"
gills, in the forms of folds on the outer sides of the gill-arches.
These are not strictly comparable to the gills of ordinary fishes,
but serve the same purpose. After a time a fold of skin grows
back over the gills, leaving only an opening to the exterior on
the left side of the body, and ultimately these organs gradually

458 ANIMAL RESPIRATION

shrivel up and the gill-clefts close, while at the same time the
lungs grow out from the under side of the gullet. Corresponding
changes take place in the heart and blood-vessels, and, as often
remarked, the successive stages in the life-history afford a practical
lesson in evolution ; for if such a startling series of changes can
take place in the life of one animal, it is not difficult to picture a
similar sequence of events in the evolution of a group.

CHAPTER XXXIX
ANIMAL RESPIRATION— AMPHIBIOUS INVERTEBRATES

As will already have been gathered from what has been said
in chapter xxxvii, numerous interesting illustrations of the amphi-
bious habit are furnished by certain members of the important
animal groups of the Mollusca and Arthropoda.

MOLLUSCS (MOLLUSCA)

ORIGIN OF LAND-SNAILS AND SLUGS. — We have seen in the
last chapter how the study of amphibious fishes throws light upon
the evolution of land vertebrates, and amphibious species are
included in typical groups of both marine and freshwater molluscs,
which enable us to understand how land-snails proper have origin-
ated from aquatic ancestors. It is extremely probable that land
molluscs have been derived from two sources, some being de-
scendants of marine forms living between tide -marks, and others
offshoots from estuarine or freshwater groups. In the latter case
we are obliged to fall back in the end upon the sea as the original
home of molluscs, for from it have been populated the estuaries
and rivers of the globe. And, as in the case of land vertebrates,
aquatic molluscs have given rise to land molluscs as the result of a
keen struggle for existence, which has driven certain forms of life
from sea to land, either directly or after a more or less prolonged
sojourn in brackish or fresh water.

Among the most interesting marine snails which afford a hint
as to one set of conditions under which amphibious habits may be
acquired are the species included in the Periwinkle Family (Lit-
torinida), plant-eating forms characteristic of the region between
tide -marks. The gill -cavity is here more or less adapted for
breathing damp air when the tide is down, and the contained
gill is reduced in size, as its chief work is done when the animal

459

460

ANIMAL RESPIRATION

is covered by water. In the case of those kinds of Periwinkle
which live high up on the shore we should naturally expect to
find the gill more reduced than in allied species living further
down, since in the former case its work would be inconsiderable,
as its owner would only be covered with water for a brief period
of time. Such an expectation is actually realized in one British
species (Littorina rudis] which lives near high- water mark, and
which has a smaller gill than the common kind (L. littorea) that

spends a larger part of its life
under water. In the former spe-
cies, too, folds richly supplied with
blood-vessels are beginning to
appear on the inner surface of
the gill-cavity (fig. 566), so as to
increase its efficiency as a lung.
After what has been said it will
/HEART not surprise the reader to hear that
there are certain tropical forms of
Periwinkle (native to Central
America and the West Indies)
which live to a considerable ex-
^•f;--R?ofofGai-cavityinaspeciesofperi- tent above high-water mark, and

winkle (Lttterma rudis), which lives near high-water

mark (enlarged). Front end is at top of figure. are eVCn found UOOtt trCCS. Such

X, Commencing net-work of lung-folds; Int., intestine;

A and v, auricle and ventricle of heart; K, kidney; CaSCS lead On tO Certain families of

as., water-testing organ (osphradium).

Snails living on land, in which the

gill has entirely disappeared, but which otherwise are probably
closely related to Periwinkles. These families are among those
known as Land Operculates, on account of the possession of an
operculum or plate by which the aperture of the shell is closed
when the animal is completely withdrawn into it. It must not be
supposed, however, that all the families of land Operculates are
related to Periwinkles, for some of them can be shown with
considerable probability to be allies of other sorts of marine
Snails.

It is quite possible that some kinds of snail which are now
adapted for a life on land, and breathe by gill -cavities which
simply serve as lungs, may have been derived from amphibious
freshwater ancestors. We know at any rate that double-breathers
of the kind actually exist, the most striking case being that of
the Apple- Snail (Ampullaria), native to both Africa and South

AMPHIBIOUS INVERTEBRATES

461

ILL

W.T.

America (fig. 567). The gill-cavity is here divided into upper
and lower parts by an imperfect partition, and a long breathing-
tube (siphon) can be protruded from the left side of the neck.
The upper part acts as a lung,
and its lining is raised into
numerous folds so as to increase
the air-breathing surface, while the
lower part contains a gill on the
right side. Semper (in Animal
Life] thus describes the way in
which the two divisions of the
breathing apparatus are used: —
" The Ampullaria uses both
organs in rapid alternation; lying
not far from the surface of the
water, it protrudes above it a
breathing siphon, and inhales air
through it; then it closes its
lungs, reopens the siphon, and
admits a stream of water through
it into the branchial cavity ". We
are here strongly reminded of
certain freshwater fishes already
described (see p. 450), which
frequently come to the surface to take air for breathing into their
swim-bladders.

We have so far dealt with the origin of only operculate land-
snails, many of which are closely related to Periwinkles and other
sea-snails belonging to the Fore-gilled (Prosobranch) marine forms
(see vol. i, p. 318). Such terrestrial molluscs, though common
enough in tropical regions, are quite different from the ordinary
land-snails and slugs with which we are familiar in this country,
and which constitute the group of Lung-Snails and Slugs (Pul-
monates), which do not possess an operculum to close the mouth
of the shell, and may even have no shell at all. Some at least of
these have probably been derived from the marine forms known
as Hind-gilled Snails and Slugs (Opisthobranchs), exemplified by
Bubble -Shells, Sea -Hares, Sea -Slugs, and many others (see
vol. i, p. 324). The chief evidence in support of this view con-
sists in the existence of a family of shore-snails (Siphonariada)

Fig. 567. — Apple-Snail (Ampullaria)

A, Upper surface ; B, vertical section across breathing-
organs (diagrammatic); W.T., water-testing organ

462 ANIMAL RESPIRATION

which are shown by their structure to be half-way between hind-
gilled snails and lung -snails. In one member of the family
(Siphonaria) the gill-cavity is partly converted into a lung and
the gill is reduced, while in an allied form (Gadinia) the gill has
gone altogether.

AMPHIBIOUS INSECTS (INSECTA)

Certain cases have already been described of insects which
live in water either when adult or during early stages of exist-
ence (see p. 439). But all the aquatic forms of the kind so far
dealt with are strictly air-breathers, either carrying about a supply
with them under water, or coming up to the surface from time to
time for the purposes of respiration. There are, however, insects
which are far more thorough-going water animals than this, being
able to breathe the air dissolved in water during the early part of
their lives. And since such forms are typical air-breathers when
adult, they are just as much entitled to be called amphibious as
frogs and newts. There is, nevertheless, a very great difference
between the ancestral history of amphibious vertebrates and am-
phibious insects. The former are terrestrial forms emerging, as
it were, from the aquatic mode of life, and dependent upon a
damp atmosphere even when adult. Amphibious insects, on the
other hand, are members of a group which is thoroughly terres-
trial, and, like birds, has even conquered the realms of air. There
can be no doubt that in the very remote past insects sprang from
aquatic ancestors, but there is no reason to suppose that these
were in the least like the water-inhabiting stages of such members
of the class as are amphibious. It is, in fact, a case of the re-
acquirement of aquatic habits in the history of a group, i.e. it is
a secondary phenomenon, while the amphibiousness of frogs and
the like is a primary phenomenon, due to the fact that they have
been specialized from fish-like creatures, which are recapitulated,
as it were, by the tadpole stage.

Amphibious insects chiefly, if not entirely, belong to one or
other of two orders, Net- winged Insects {Neuropterd) and Two-
winged Insects (Dipterd), which it will be most convenient to
consider separately.

AMPHIBIOUS INVERTEBRATES 463

AMPHIBIOUS NET-WINGED INSECTS (NEUROPTERA)

The most remarkable structures by which amphibious insects
are enabled to breathe the air dissolved in water are those known
as tracheal gills, which may be briefly defined as gills traversed
by air-tubes (trachea) that do not open to the exterior. Gills of
this sort are found in the early stages of many net-winged insects,
but are rare in the adults. They are clearly of secondary nature,
i.e. do not correspond to the gills of an ancient ancestral stock, for
the air-tubes which they contain are structures evolved with refer-
ence to air-breathing (see p. 434), and are here pressed into the
service, as it were, of breathing in water. The gill itself is an
outgrowth of the body which offers a large surface for exchange
of gases between the air in the air-tubes and that dissolved in
the surrounding water. The evolution of an aquatic mode of life
requires a very long time, especially when it takes place in so
typical a class of land animals as insects, and even without definite
evidence it would be reasonable to suppose that the amphibious-
ness of certain insects has taken longer to come about than the
acquirement of an aquatic habit by the insects, elsewhere described,
which are not able to breathe air dissolved in water. There is,
however, positive geological evidence to show that the tracheal
gills of net-winged insects are structures of very great antiquity.
Numerous fossils belonging to this group have been found in
rocks belonging to that immensely remote period to which the
name Carboniferous has been applied because its luxuriant vege-
tation has been converted into those coal-deposits which are of
greatest importance. Many of the extinct insects of that period
belong to existing groups of Net -Wings, while others, though
referable to this order, constitute groups which have no living
representatives. One of these insects (Corydaloides Scudderi)
is especially interesting, for, when adult, it possessed tracheal
gills resembling those found in the early stages of May- Flies,
such as will be described later on.

We will now consider the amphibious members of certain
families of Net-winged Insects. These are: Stone-Flies (Per-
lidae), Dragon-Flies (Odonata), May-Flies (Ephemeridse), Alder-
Flies (Sialidae), and Caddis-Flies (Phryganeidae).

Stone-Flies (Per lido). — These insects, of which about two
dozen British species, are known, live in rapidly-flowing streams

404

ANIMAL RESPIRATION

during the early part of their existence, and, although land-animals
when adult, are always found in the neighbourhood of water.
There is no quiescent or pupa stage in the life-history, but the
young Stone- Fly, when hatched from the egg, closely resembles
the adult, except that wings are absent. It is usual to apply the
term nymph to a stage of this sort, reserving the word larva for
caterpillars, grubs, and the like, that are very unlike the perfect
insects which they become after passing through a more or less
quiescent pupal condition.

The nymphs of Stone- Flies possess numerous air-tubes, but
these do not open to the exterior by means of spiracles. Breath-
ing is either effected through the skin,
which is very thin at certain spots, or by
means of tracheal gills, which differ in
shape and position. These gills persist
in the adult (as, e.g. in Pteronarcys\
though they often become reduced in
size, and it is not known how far they
are of any use (fig. 568). That they
should persist at all is very interesting in
view of the fact that in certain extinct
forms (see p. 463) they were possessed
by the perfect insect. We know that
the coal-plants, among which these an-
cient insects lived, grew in jungles and
swamps where the air was very damp,
and this probably favoured the retention
of tracheal gills throughout life.
Dragon- Flies (Odonatd). — A female dragon-fly lays her eggs
either in the water or upon the stem of a water-plant, and they
hatch out into flattened wingless nymphs, which possess an
elaborate system of air-tubes that probably do not communicate
with the exterior. The way in which such nymphs breathe is
not always the same, but the commonest, and at the same time
the most interesting, is by means of rectal gills, so called because
they are folds in the lining of the last part of the intestine
(rectum]. These folds are either plate-like or in the form of
small finger-shaped projections, but in either case they are richly
provided with air -tubes, and collectively possess a very large
breathing surface (fig. 569). The rectum alternately contracts

Fig. 568.— A Stone-Fly (Pteronarcys]
A, Under side of adult, with limbs cut
short ; gggg, reduced tracheal gill-tufts
(enlarged); B, a gill -tuft (more highly
magnified).

AMPHIBIOUS INVERTEBRATES

465

and dilates, so that water is forced out and drawn in rhythmically.
When the nymph develops into the adult dragon-fly, spiracles are
acquired by the system of air-tubes, the water is abandoned, and
ordinary air is breathed in the manner usual among insects.

May -Flies (Ephemerida}. — The adult insects are here thor-
oughly aerial, as in the groups of Net- Wings so far described,
but the nymphs present a maximum amount of adaptation to
an aquatic mode of life. They possess
tracheal gills of various kind, a typical case
being that of the Common May- Fly (Ephe-
mera v^llgata), in which these structures are
in the form of a double
series of tufted out-
growths running along V1^z

i.ap.'
Fig. 569.— Dragon-Fly Nymph

Dissection to show air-tubes (£/)
which supply rectal gills; int., intes-
tine (rectum); i.ap., intestinal aper-
ture. Enlarged.

Fig- 570.— Nymph of
Common May - Fly
(Ephemera vulgatei),
enlarged, to show
tracheal gills at sides
and three tail-rods.

Fig- 571- — Hinder part
of Nymph of a May-
Fly (Cloeon dipterum],
enlarged. A, Last three
segments and bases of
tail-rods. Last cham-
ber (L.CH.) of heart gives
off vessels to the tail-
rods. Arrows indicate
direction of blood-flow.
B, Part of a tail-rod,
showing perforated walls
of central blood-vessel.

the abdomen, and traversed by numerous air-tubes (fig. 570).
Breathing is also helped, it would appear, by three feathery rods
which project from the hinder end of the body, and which differ
greatly both in structure and mode of action from tracheal gills.
The arrangement has been worked out in detail in the nymph of
one kind of May-Fly (Cloeon dipteruni). As in Insects generally,
the heart is here a slender tube situated in the middle line close
below the upper surface of the body. It consists of a series of
chambers which receive blood by means of paired valvular aper-
tures and pump it forwards. To this direction of blood-flow there

VOL. II.

466

ANIMAL RESPIRATION

is, however, one exception in this particular case, for part of the
blood from the last chamber but one passes back through a pair of
valves into the last chamber, which forces it into the three tail- rods
through a corresponding number of vessels (fig. 571). Each
of these vessels is perforated by a series of oval holes through
which the blood escapes into the rods, which are hollow, and
it is then returned to the body, presumably in a purified condi-
tion, having absorbed some of the dissolved oxygen from the
surrounding water and got rid of waste carbonic acid gas.

We have noted in a previous chapter that the delicate gills
of such creatures as fishes and the higher crustaceans are pro-
tected by being placed in special gill-cavities. Tracheal gills may
also be sheltered in various ways. They some-
times grow out from the under sides of firm plates
(Oligoneuria), and in other cases the gills of either
side are shielded by a flat gill -cover ( Tricorythus\
But the most remarkable instance is afforded by
the nymphs of certain May- Flies (species of
Prosopistomd) which are native to Europe, West
Africa, and Madagascar. The nymph, which lives
in swift streamlets, looks like, and was formerly
mistaken for, one of the lower crustaceans (fig.
572). The head and front part of the abdomen
are covered by a sort of shield, under which there
is a gill-cavity containing five pairs of tracheal
gills. Water flows into the cavity through an
opening on either side, and out again by a single
aperture at the back. Sharp (to whose admirable
volumes on Insects in The Cambridge Natural History the present
writer is here and elsewhere greatly indebted) says of this nymph
that it is " more completely adapted for an aquatic life than any
other insect at present known ".

It only remains to mention that the nymphs of some May-
Flies appear to partly breathe by means of the rectum, like the
corresponding stages of Dragon- Flies which have been already
described.

Alder-Flies (Sialidtz). — The Alder-Fly (Sialis lutaria] is a
very common British insect which haunts the banks of sluggish
streams. The eggs are laid in regular rows on rushes or other
plants which grow near the water. The aquatic young which

Fig. 572. — Crustacean-
like Nymph of a May- Fly
(Prosopistoma), enlarged.

AMPHIBIOUS INVERTEBRATES 467

hatch out from them may be called larvae (not nymphs), because
they differ considerably from the adult, into which they do not
pass directly, there being an intermediate quiescent or pupa stage.
Each of the first seven rings of the abdomen bears a pair of
tracheal gills in the form of jointed threads, and an unjointed rod
projects backwards from the tip of the tail. The gills are kept in
constant motion, so that the water in their vicinity is continually
renewed.

Caddis- Flies (Phryganeidce). — An adult Caddis- Fly has four
somewhat hairy wings, giving it some resemblance to a moth;
indeed, it is sometimes called a Caddis-" Moth ". The eggs are
laid in water, and from them elongated larvae hatch out, which
are familiarly known as " caddis- worms ", and construct for them-
selves protective cases of such materials as may be at hand.
The cases of the commonest British species are straight or
slightly curved tubes, with a large opening at the head-end and
a smaller one at the tip. The breathing organs of the larva
are thread-like tracheal gills borne by the abdomen, upon the
upper side of which are also a number of thin places which
possibly assist in respiration by permitting diffusion of gases.
The gills are kept moving, and water continually streams through
the case, entering at its hinder end.

AMPHIBIOUS TWO-WINGED INSECTS (DIPTERA)

In two families of this order the early stages of existence
are fully adapted to life in water, while the adults are aerial and
breathe ordinary air by means of a complex set of air-tubes.
These are Midges (Ckironomida) and Sand- Midges (Simuliada).

Midges (Chironomidte). — A very large number of Midges are
included in the genus (Chironomus] from which this family takes its
name. The eggs are laid in water, and hatch out into active wrig-
gling larvae, which are commonly of a red colour, and are then
known as " blood- worms ". This hue is caused by the presence of
a pigment (hemoglobin) identical with that contained in blood, and
which is of great importance in breathing, since it readily takes up »
oxygen from the surrounding medium, easily parting with it again
to the body. Some of these larvae live at considerable depths
in lakes (1000 feet in Lake Superior) or even in the sea, and
the amount of red colouring-matter they possess is in direct pro -

468

ANIMAL RESPIRATION

portion to the depth at which they are able to exist, while at the
same time their air-tubes are more and more feebly developed.
Some of them, on the other hand, are altogether destitute of
red pigment and live near the surface, being at the same time
well provided with air-tubes. The larvae may breathe entirely
through the skin, but are usually provided with thread-like gills
on the eleventh and twelfth rings of the body. After a time
the midge-larva passes into a quiescent pupa-stage, during which
breathing is effected by means of a pair of tufted gills situated at
the front end of the body. These contrast markedly with the two
breathing trumpets which the pupa of an ordinary gnat bears in
a similar position (see p. 442), and which admit ordinary air into
the breathing-tubes.

Sand-Midges (Simuliadcz). — These notorious blood-suckers are
found in most parts of the world, sometimes occurring in vast

swarms, and bestowing their
unwelcome attentions on both
human beings and cattle. The
cylindrical larva (fig. 573)
possesses a sucker at the hin-
der end of its body, by means
of which it holds on to a
stone or other firm body, and
is maintained in a vertical
position. Two beautiful tufted
gills are borne upon the head
and are kept in constant move-
ment, which serves a double
purpose, promoting respira-
tion and also bringing a supply
of food within reach of their
owner. Later on the larva
becomes a pupa, which is
contained in a sort of open

case, from which the head end freely projects. Upon this are
carried two bunches of long tubular gills (fig. 573), by which
breathing is carried on until the perfect insect is fully formed and
ready to make its escape into the air.

Fig. 573. — Stages in life-history of a Sand-Midge (Simulia),
enlarged (actual size shown by short lines). A, Larva. B, Pupa.
c, Pupa-case.

AMPHIBIOUS INVERTEBRATES

469

AMPHIBIOUS CRUSTACEANS (CRUSTACEA)

Some mention has already been made of Land-Crabs (see
p. 443) as descended from shore -dwelling forms which have
lost their gills to a greater or less extent, and converted their
gill-chambers into organs capable of breathing ordinary air. It
would not therefore be surprising to find Crabs of truly amphi-
bious nature forming a sort of half-way house between purely
marine and purely terrestrial forms. Such a condition is actually
realized by certain shore-haunting Crabs belonging to a widely-
distributed family (Grapsida). The gills, though reduced in size,
are still useful, and the gill-chamber possesses folds and ridges
on its lining which enable it to be used as a lung. Such Crabs,
when out of water, have been observed to lift up the hinder part
of the body for the purpose of admitting air into the gill-chambers
from the back. There is also an aperture at the front end of each
chamber which is supposed to serve for the entry of water to be
used by the gills.

Some account has been given in a preceding section (see
p. 220) of the habits of the Robber-Crab or Palm-Thief (Birgus

A.L.V.

6.L.V.

GIL

E.L.V.

Fig. 574.— Diagrammatic Cross Section through Breathing Organs of Robber Crab

(Birgus latro]

A.L.v., Vessels carrying impure blood to lung-folds; E.L.V., vessels bringing pure blood from
lung- folds; and E.L.V'., opening of one of these into blood-space surrounding heart (pericardial
space); E.G.V., vessels returning pure blood from gills to pericardial space.

latro] of the Cocos-Keeling Islands. The breathing organs of
this curious creature, which is really a Hermit-Crab that has
taken to live on land, are in a very interesting condition (fig.
574). The gills are much reduced in size, and the gill-chambers
are divided into a small lower part for their reception, and a very
much larger upper part acting as a lung, and having its lining
raised into an elaborate system of folds. How far this animal is

470 ANIMAL RESPIRATION

really amphibious is rather difficult to say, for though it some-
times visits the sea, it does not of necessity follow that its gills
are then used for breathing air dissolved in water, though this
would seem probable. The division of each gill-chamber into two
distinct parts brings to mind the arrangement found in the amphi-
bious Apple- Snail (see p. 461).

END OF VOL. II

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The Animal Life of the w°rld in its
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Its Nature and Uses", &c. With contributions from many specialists. Illustrated by a
series of about 100 separately-printed plates and 1000 figures in the text. In 8 divisional
volumes, super-royal quarto, handsomely bound in cloth, with cover design by Mr. TALWIN
MORRIS, price 7.?. 6d. net each. In complete sets only.

In preparing THE MODERN CARPENTER the editor has had the great advantage of working upon
the basis of Newlands's Carpenter and Joiner s Assistant, which for nearly half a century has been
accepted as a standard authority on the subjects of which it treats, and for many years has been
recommended by the Royal Institute of British Architects as a text-book for the examination of that
society. And yet in the present work it has been possible to preserve only a very small part of
Newlands's treatise, invaluable though this has been to two generations of craftsmen. While the
fundamental features of arrangement and method which distinguish this famous work have been
retained, the matter has had to be entirely rewritten, and many new sections have been added, on
subjects not touched upon in the older work, with which the carpenter of the present day requires to be
familiar.

In the new book, indeed, the old foundations that have stood the test of half a century of practical use
have been retained, but the superstructure is wholly new.

The lesson to be learned from this fact is not far to seek. It is that the modern carpenter requires a
far wider expert knowledge than sufficed his predecessor. The development of wood-working
machinery, the introduction of new kinds of timber, improvements in the design of structures, the more
thorough testing of timbers, and progress in the various industries with which Carpentry, Joinery, and
Cabinet-making are intimately allied, have all helped to render the craft more complex. The carpenter
of the present day has no use for the old "rule of thumb" methods; his calling is both an art and a
science, and knowledge, knowledge, and again knowledge is the primary condition of success.

The editor of THE MODERN CARPENTER, Mr. G. Lister Sutcliffe, Associate of the Royal Institute
of Architects, needs no introduction to practical men; his name is already well known not only
through his professional position in the architectural world, but through his editorship of Modern House-
Construction, a work which, although issued only a few years ago, has already become a standard book
of reference. Mr. SUTCLIFFE'S large experience has enabled him to enlist the services of a highly-
qualified staff of experts, whose special knowledge, acquired through long years of practical work, is
now placed at the disposal of every member of the craft. The first condition in selecting the contri-
butors to the work was that they should be practical men, not only possessing the indispensable
knowledge, but having the ability to impart it. The result is that within the eight divisional-volumes of
this work we have a treatise on every branch of the craft, distinguished by four outstanding qualities : —
It is (i) complete, (2) clear, (3) practical, and (4) up-to-date.

An idea of the scope of THE MODERN CARPENTER may be gathered from the fact that while its
predecessor, The Carpenter and Joiner s Assistant, comprised only eight sections, the new work
includes no fewer than sixteen. A glance at these will show that the work covers the whole field ;
it is a complete encyclopaedia upon every subject that bears upon the everyday work of the practical man.

I. Styles of Architecture.
II. Woods: Their Characteristics and Uses.

III. Wood-working Tools and Machinery.

IV. Drawing and Drawing Instruments.
V. Practical Geometry.

VI. Strength of Timber and Timber Framing.
VII. Carpentry.
VIII. Joinery and Ironmongery.

IX. Staircases and Handrailing.
X. Air-tight Case-Making.
XI. Cabinet-Making.
XII. Wood-Carving.

XIII. Shop Management.

XIV. Estimating.
XV. Building Law.

XVI. Index, Glossary, &e.

The Illustrations are not the least of the many notable features of this great undertaking. The work
is embellished in the first place with about 100 full-page plates, reproduced, some in colours, by the
most approved processes of mechanical engraving, and printed on specially-prepared paper. In addition
to this unique collection there are no fewer than 1000 diagrams and designs in the body of the work.
No trouble or expense has indeed been spared to procure illustrations where these could elucidate the
text.

Prospectus of any Book post free.

The Gresham Publishing Company.

The ImPerial Edition of the Novels of CHARLES
DICKENS, in 15 volumes, large square 8vo, cloth
extra, gilt top, price 4J. 6d. net each volume.

An Ideal Issue. ONE NOVEL, ONE VOLUME. Despite
varying lengths, the paper, &c., is so adjusted that each volume
is uniform in thickness and size.

The Cheapest Edition. The price of each volume is 4.$-. 6d.
net, making the edition the cheapest of the best editions.

Sumptuously Bound. The cloth is of the finest and is im-
perial red in colour. The embellishments (produced in gold)
are an appropriate design of national arms and imperial em-
blems by the eminent designer, Talwin Morris.

Illustrations a Unique Feature. Every picture drawn spe-
cially at enormous cost for this "Imperial" edition by the best
known and most celebrated Artists of to-day.

George Gissing's Masterly Study. A literary character
study, the work of this great authority, forms one of the volumes
of this issue, and is illustrated with pictures of some of the
quaint old hostelries and places made famous by Dickens, and
is altogether an invaluable addition to this issue.

Presentation Portrait. To every subscriber to this edition
will be presented with the last volume a magnificent Photo-
gravure of Charles Dickens. It is printed on the finest plate
paper, 22 inches by 30 inches, and has been specially engraved
for this edition.

A List of the Novels.

The following is a list of the volumes in the Imperial Edition: —

r. The Pickwick Papers.

2. Oliver Twist.

3. Nicholas Nickleby.

4. Martin Chuzzlewit.

5. The Old Curiosity Shop

6. Barnaby Rudge.

7. David Copperfield.

8. Bleak House.

9. Sketches by Boz.

10. Hard Times and Master Humphrey's Clock.

11. Christmas Books.

12. Dombey and Son.

13. Little Dorrit.

14. A Tale of Two Cities.

15. Charles Dickens: A Critical Study.

By GEORGE GISSING.

Prospectus of any Book post free.

301521

U8RARY

UNIVERSITY OF CALIFORNIA LIBRARY

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